diff --git a/src/main/orcad/orcad-data-root-locality.test.ts b/src/main/orcad/orcad-data-root-locality.test.ts
new file mode 100644
index 00000000000..821fd7623ff
--- /dev/null
+++ b/src/main/orcad/orcad-data-root-locality.test.ts
@@ -0,0 +1,27 @@
+import { mkdtempSync, rmSync } from 'node:fs'
+import { tmpdir } from 'node:os'
+import { join } from 'node:path'
+import { describe, expect, it } from 'vitest'
+import { orcadDataRootIsOnLocalDisk } from './orcad-data-root-locality'
+
+describe('orcadDataRootIsOnLocalDisk', () => {
+  it.runIf(process.platform === 'linux' || process.platform === 'darwin')(
+    'recognises a local temp directory, and refuses a path it cannot inspect',
+    () => {
+      const root = mkdtempSync(join(tmpdir(), 'orcad-locality-'))
+      try {
+        expect(orcadDataRootIsOnLocalDisk(root)).toBe(true)
+      } finally {
+        rmSync(root, { recursive: true, force: true })
+      }
+      expect(orcadDataRootIsOnLocalDisk(join(root, 'missing'))).toBe(false)
+    }
+  )
+
+  it('treats a Windows UNC data root as reachable from other machines', () => {
+    expect(orcadDataRootIsOnLocalDisk('\\\\filer\\home\\orca', 'win32')).toBe(false)
+    expect(orcadDataRootIsOnLocalDisk('\\\\?\\UNC\\filer\\home\\orca', 'win32')).toBe(false)
+    expect(orcadDataRootIsOnLocalDisk('C:\\Users\\u\\AppData\\Roaming\\orca', 'win32')).toBe(true)
+    expect(orcadDataRootIsOnLocalDisk('\\\\?\\C:\\Users\\u\\orca', 'win32')).toBe(true)
+  })
+})
diff --git a/src/main/orcad/orcad-data-root-locality.ts b/src/main/orcad/orcad-data-root-locality.ts
new file mode 100644
index 00000000000..064ba958864
--- /dev/null
+++ b/src/main/orcad/orcad-data-root-locality.ts
@@ -0,0 +1,49 @@
+/**
+ * Whether a data root sits on a disk only this machine mounts. An instance-lock record that names
+ * no machine is judged by its PID only when this holds: on a network home another machine may be
+ * running it, and a PID is evidence only on the machine that issued it.
+ */
+import { statfsSync } from 'node:fs'
+import process from 'node:process'
+
+// Linux `statfs` f_type magics (linux/magic.h) of local disk and memory filesystems.
+const LINUX_LOCAL_FILESYSTEM_MAGICS = new Set([
+  0xef53, // ext2/3/4
+  0x58465342, // xfs
+  0x9123683e, // btrfs
+  0x2fc12fc1, // zfs
+  0xca451a4e, // bcachefs
+  0xf2f52010, // f2fs
+  0x3153464a, // jfs
+  0x52654973, // reiserfs
+  0x3434, // nilfs
+  0x01021994, // tmpfs
+  0x858458f6, // ramfs
+  0x794c7630, // overlayfs
+  0xf15f, // ecryptfs
+  0x4d44, // vfat
+  0x2011bab0, // exfat
+  0x482b // hfsplus
+])
+
+export function orcadDataRootIsOnLocalDisk(
+  dataRoot: string,
+  platform: NodeJS.Platform = process.platform
+): boolean {
+  if (platform === 'win32') {
+    // `\\?\C:\` and `\\.\C:\` name local drives; any other `\\` path is a share. Mapped network
+    // drives look local here, but every Windows writer records its MachineGuid anyway.
+    return !/^[\\/]{2}/u.test(dataRoot) || /^[\\/]{2}[?.][\\/][a-z]:/iu.test(dataRoot)
+  }
+  try {
+    const type = statfsSync(dataRoot).type
+    if (platform === 'linux') {
+      return LINUX_LOCAL_FILESYSTEM_MAGICS.has(type)
+    }
+    // macOS numbers filesystem types as their drivers load, so compare against the boot volume's,
+    // which is always a local disk; network filesystems never share its type.
+    return platform === 'darwin' && type === statfsSync('/').type
+  } catch {
+    return false
+  }
+}
diff --git a/src/main/orcad/orcad-instance-lock-reclaim.ts b/src/main/orcad/orcad-instance-lock-reclaim.ts
new file mode 100644
index 00000000000..f674f971b14
--- /dev/null
+++ b/src/main/orcad/orcad-instance-lock-reclaim.ts
@@ -0,0 +1,253 @@
+/**
+ * Replacing a stale orcad instance lock without ever displacing a live one (see
+ * `orcad-instance-lock.ts`).
+ */
+import { createHash, randomUUID } from 'node:crypto'
+import {
+  existsSync,
+  linkSync,
+  lstatSync,
+  mkdirSync,
+  readdirSync,
+  renameSync,
+  rmSync,
+  unlinkSync,
+  writeFileSync
+} from 'node:fs'
+import { basename, dirname, join } from 'node:path'
+import process from 'node:process'
+import { OrcadInstanceLockError } from './orcad-data-root-privacy'
+import { readNodeFileSyncWithinLimit } from '../../shared/node-bounded-file-reader'
+import { hasErrorCode } from '../daemon/daemon-process-inspection'
+
+export const MAX_ORCAD_LOCK_BYTES = 64 * 1024
+// Serializes stale-lock recovery, so no reclaimer ever renames away a lock another just published.
+const RECOVERY_DIR_SUFFIX = '.recovery'
+const RECOVERY_OWNER_FILE = 'owner'
+const RECOVERY_RELEASED_FILE = 'released'
+
+/** Who is recovering, as a lock record, and whether a recovery owner's holder provably exited. */
+export type OrcadLockRecoveryOwner = {
+  record: string
+  exited: (ownerRecord: string) => boolean
+}
+
+/**
+ * Why rename-and-then-publish rather than unlink-and-write: rename claims one exact directory
+ * entry, so a replacement written between our read and our write stays at the canonical path
+ * and wins — we never delete a record we did not inspect.
+ */
+export function reclaimAndPublish(
+  lockPath: string,
+  dataRoot: string,
+  inspectedContents: string,
+  publish: () => boolean,
+  owner: OrcadLockRecoveryOwner
+): void {
+  return withRecoveryLock(lockPath, inspectedContents, owner, () => {
+    // Another reclaimer finished first: whatever it published is authoritative, not stale.
+    if (readBoundedLockFile(lockPath) !== inspectedContents) {
+      throw new OrcadInstanceLockError(
+        'orcad_instance_lock_held',
+        `Another process took the data root ${dataRoot} while this one was reclaiming a stale lock.`
+      )
+    }
+    renameAndPublish(lockPath, dataRoot, inspectedContents, publish)
+  })
+}
+
+function renameAndPublish(
+  lockPath: string,
+  dataRoot: string,
+  inspectedContents: string,
+  publish: () => boolean
+): void {
+  const claimPath = `${lockPath}.stale-${process.pid}-${randomUUID()}`
+  try {
+    renameSync(lockPath, claimPath)
+  } catch {
+    throw new OrcadInstanceLockError(
+      'orcad_instance_lock_held',
+      `Could not reclaim the stale orcad instance lock at ${lockPath}; another process is ` +
+        'holding it. Retry, or stop the other orcad.'
+    )
+  }
+  // A contender may have replaced the entry after the liveness check; never displace its record.
+  const claimedContents = readBoundedLockFile(claimPath)
+  if (claimedContents !== inspectedContents) {
+    restoreDisplacedLock(claimPath, lockPath, claimedContents)
+    throw new OrcadInstanceLockError(
+      'orcad_instance_lock_held',
+      `The orcad instance lock at ${lockPath} changed while reclaiming a stale record.`
+    )
+  }
+  const published = publish()
+  // A uniquely named claim is inert either way.
+  unlinkQuietly(claimPath)
+  if (!published) {
+    // Someone else claimed it first. Their record is authoritative; ours is not.
+    throw new OrcadInstanceLockError(
+      'orcad_instance_lock_held',
+      `Another orcad took the data root ${dataRoot} while this one was reclaiming a stale lock.`
+    )
+  }
+}
+
+/**
+ * One recovery lock per stale record (named by its contents), in generations: a dead
+ * owner's generation is never removed or renamed, the next one is created beside it. Taking one is
+ * exclusive, and a later generation exists only once every earlier owner provably exited, so the
+ * newest generation's owner is the only live reclaimer and no live owner is ever displaced.
+ */
+function withRecoveryLock<T>(
+  lockPath: string,
+  inspectedContents: string,
+  owner: OrcadLockRecoveryOwner,
+  recover: () => T
+): T {
+  const prefix = orcadLockRecoveryPrefix(lockPath, inspectedContents)
+  const generations = recoveryGenerations(prefix)
+  const newest = generations.at(-1)
+  const newestDir = `${prefix}${newest}`
+  const newestOwner = newest === undefined ? null : readRecoveryOwner(newestDir)
+  const free =
+    newestOwner === null ||
+    // A generation is never published without its owner; one without is a cut-short cleanup's.
+    newestOwner.state === 'absent' ||
+    (newestOwner.state === 'present' &&
+      (existsSync(join(newestDir, RECOVERY_RELEASED_FILE)) || owner.exited(newestOwner.record)))
+  const generation = newest === undefined ? 0 : newest + 1
+  if (!free || !takeRecoveryLock(`${prefix}${generation}`, owner.record)) {
+    throw new OrcadInstanceLockError(
+      'orcad_instance_lock_held',
+      `Another process is reclaiming the stale orcad instance lock at ${lockPath}. Retry.`
+    )
+  }
+  try {
+    return recover()
+  } finally {
+    const current = readBoundedLockFile(lockPath)
+    if (current === null || current === inspectedContents) {
+      // Kept, marked released: a reused generation number could be judged by a contender that
+      // read this generation's owner before we let go.
+      writeQuietly(join(`${prefix}${generation}`, RECOVERY_RELEASED_FILE))
+    } else {
+      // Once the stale record is gone no reclaimer can use its recovery locks again.
+      for (const stale of recoveryGenerations(prefix)) {
+        try {
+          rmSync(`${prefix}${stale}`, { recursive: true, force: true })
+        } catch {
+          // A late contender writing into it; it is inert once the stale record is gone.
+        }
+      }
+    }
+  }
+}
+
+/**
+ * Keyed by contents alone: everyone who inspected the same stale record must share one recovery
+ * lock, whatever the path held when they looked again (another inode, or nothing mid-reclaim).
+ */
+export function orcadLockRecoveryPrefix(lockPath: string, inspectedContents: string): string {
+  const key = createHash('sha256').update(inspectedContents).digest('hex')
+  return `${lockPath}${RECOVERY_DIR_SUFFIX}-${key.slice(0, 16)}-`
+}
+
+function recoveryGenerations(prefix: string): number[] {
+  const name = basename(prefix)
+  try {
+    return readdirSync(dirname(prefix))
+      .filter((entry) => entry.startsWith(name) && /^\d{1,9}$/u.test(entry.slice(name.length)))
+      .map((entry) => Number(entry.slice(name.length)))
+      .sort((a, b) => a - b)
+  } catch {
+    return []
+  }
+}
+
+/**
+ * Staged with its owner, then renamed into place: a recovery lock never exists without a complete
+ * owner, and a rename onto one that does (a non-empty directory) fails instead of replacing it.
+ */
+function takeRecoveryLock(dir: string, ownerRecord: string): boolean {
+  const staged = `${dir}.staged-${process.pid}-${randomUUID()}`
+  try {
+    mkdirSync(staged, { mode: 0o700 })
+    writeFileSync(join(staged, RECOVERY_OWNER_FILE), ownerRecord, { flag: 'wx', mode: 0o600 })
+  } catch (error) {
+    rmSync(staged, { recursive: true, force: true })
+    throw new OrcadInstanceLockError(
+      'orcad_data_root_unusable',
+      `Cannot create ${staged}: ${error instanceof Error ? error.message : String(error)}`
+    )
+  }
+  try {
+    renameSync(staged, dir)
+    return true
+  } catch {
+    // Whatever holds the name, it was not created by this attempt; only our staging is removed.
+    rmSync(staged, { recursive: true, force: true })
+    return false
+  }
+}
+
+/** No-clobber restore: a third contender's newer record at the canonical path stays authoritative. */
+function restoreDisplacedLock(
+  claimPath: string,
+  lockPath: string,
+  claimedContents: string | null
+): void {
+  try {
+    linkSync(claimPath, lockPath)
+    unlinkSync(claimPath)
+  } catch {
+    if (claimedContents === null) {
+      return
+    }
+    try {
+      writeFileSync(lockPath, claimedContents, { flag: 'wx', mode: 0o600 })
+      unlinkSync(claimPath)
+    } catch {
+      // A newer contender won, or restoration is unavailable; fail closed.
+    }
+  }
+}
+
+/** An unreadable owner fails closed: it proves nothing about whether its reclaimer exited. */
+function readRecoveryOwner(
+  dir: string
+): { state: 'absent' } | { state: 'present'; record: string } | { state: 'unreadable' } {
+  const path = join(dir, RECOVERY_OWNER_FILE)
+  try {
+    lstatSync(path)
+  } catch (error) {
+    return hasErrorCode(error, 'ENOENT') ? { state: 'absent' } : { state: 'unreadable' }
+  }
+  const record = readBoundedLockFile(path)
+  return record === null ? { state: 'unreadable' } : { state: 'present', record }
+}
+
+function writeQuietly(path: string): void {
+  try {
+    writeFileSync(path, '', { flag: 'wx', mode: 0o600 })
+  } catch {
+    // Unwritable: the owner record still releases it once this process exits.
+  }
+}
+
+export function unlinkQuietly(path: string): void {
+  try {
+    unlinkSync(path)
+  } catch {
+    // Already gone, or inert under its unique name.
+  }
+}
+
+export function readBoundedLockFile(path: string): string | null {
+  try {
+    const { buffer, stats } = readNodeFileSyncWithinLimit(path, MAX_ORCAD_LOCK_BYTES)
+    return stats.isFile() ? buffer.toString('utf8') : null
+  } catch {
+    return null
+  }
+}
diff --git a/src/main/orcad/orcad-instance-lock.test.ts b/src/main/orcad/orcad-instance-lock.test.ts
index 29db0a010bd..305cf0bd5bc 100644
--- a/src/main/orcad/orcad-instance-lock.test.ts
+++ b/src/main/orcad/orcad-instance-lock.test.ts
@@ -12,7 +12,8 @@ import {
 } from 'node:fs'
 import { tmpdir } from 'node:os'
 import { join } from 'node:path'
-import { afterEach, describe, expect, it } from 'vitest'
+import { afterEach, describe, expect, it, vi } from 'vitest'
+import type * as NodeFs from 'node:fs'
 import { z } from 'zod'
 import {
   acquireOrcadInstanceLock,
@@ -22,6 +23,31 @@ import {
   type OrcadLockRecord
 } from './orcad-instance-lock'
 import { OrcadInstanceLockError } from './orcad-data-root-privacy'
+import { orcadLockRecoveryPrefix } from './orcad-instance-lock-reclaim'
+
+// Runs once, right after the next rename out of the canonical lock path: a contender's turn.
+const afterLockRename = vi.hoisted((): { next: (() => void) | null; failNext: boolean } => ({
+  next: null,
+  failNext: false
+}))
+vi.mock('node:fs', async (importOriginal) => {
+  const actual = await importOriginal<typeof NodeFs>()
+  return {
+    ...actual,
+    renameSync: (from: NodeFs.PathLike, to: NodeFs.PathLike) => {
+      if (afterLockRename.failNext && String(from).endsWith('orcad.lock')) {
+        afterLockRename.failNext = false
+        throw Object.assign(new Error('EBUSY: resource busy'), { code: 'EBUSY' })
+      }
+      actual.renameSync(from, to)
+      const contender = afterLockRename.next
+      if (contender && String(from).endsWith('orcad.lock')) {
+        afterLockRename.next = null
+        contender()
+      }
+    }
+  }
+})
 
 const roots: string[] = []
 
@@ -40,6 +66,8 @@ function hooks(overrides: OrcadInstanceLockHooks = {}): OrcadInstanceLockHooks {
     incarnation: () => null,
     holderIdentity: () => 'matches',
     processIsAlive: () => false,
+    machineIdentity: () => 'machine-a',
+    dataRootIsOnLocalDisk: () => true,
     ...overrides
   }
 }
@@ -57,6 +85,8 @@ function persistedRecord(overrides: Partial<OrcadLockRecord> = {}): OrcadLockRec
 }
 
 afterEach(() => {
+  afterLockRename.next = null
+  afterLockRename.failNext = false
   for (const root of roots.splice(0)) {
     rmSync(root, { recursive: true, force: true })
   }
@@ -456,6 +486,205 @@ describe('acquireOrcadInstanceLock', () => {
     lock.release()
   })
 
+  it('never judges a holder on another machine by a PID this machine issued', () => {
+    const root = makeRoot()
+    const lockPath = join(root, ORCAD_LOCK_FILE_NAME)
+    // Machine B's orcad holds a data root on a shared home; its PID does not exist here.
+    const remote = persistedRecord({ nonce: 'machine-b', machineIdentity: 'machine-b' })
+    writeFileSync(lockPath, JSON.stringify(remote))
+    for (const role of ['orcad', 'desktop'] as const) {
+      expect(() => acquireOrcadInstanceLock(root, hooks({ role }))).toThrow(
+        expect.objectContaining({
+          code: 'orcad_instance_lock_held',
+          message: expect.stringContaining('from another machine')
+        })
+      )
+    }
+    expect(readOrcadInstanceLockRecord(lockPath)?.nonce).toBe('machine-b')
+    expect(readdirSync(root)).toEqual([ORCAD_LOCK_FILE_NAME])
+
+    // The same machine's crashed holder is still reclaimed.
+    writeFileSync(lockPath, JSON.stringify({ ...remote, machineIdentity: 'machine-a' }))
+    const lock = acquireOrcadInstanceLock(root, hooks())
+    expect(readOrcadInstanceLockRecord(lockPath)).toMatchObject({
+      nonce: lock.record.nonce,
+      machineIdentity: 'machine-a'
+    })
+  })
+
+  it('reclaims a record naming no machine only when it is provably from this one', () => {
+    const root = makeRoot()
+    const lockPath = join(root, ORCAD_LOCK_FILE_NAME)
+    // An older writer's record, on a data root other machines may share.
+    writeFileSync(lockPath, JSON.stringify(persistedRecord({ bootId: 'boot-1' })))
+    const shared = { dataRootIsOnLocalDisk: () => false }
+    expect(() => acquireOrcadInstanceLock(root, hooks(shared))).toThrow(
+      expect.objectContaining({ code: 'orcad_instance_lock_held' })
+    )
+    expect(() =>
+      acquireOrcadInstanceLock(root, hooks({ ...shared, machineIdentity: () => null }))
+    ).toThrow(expect.objectContaining({ code: 'orcad_instance_lock_held' }))
+    expect(readOrcadInstanceLockRecord(lockPath)?.nonce).toBe('stale')
+
+    // The same kernel boot is this machine.
+    const sameBoot = hooks({
+      ...shared,
+      incarnation: () => ({ linuxStartTicks: '1', bootId: 'boot-1' })
+    })
+    acquireOrcadInstanceLock(root, sameBoot).release()
+    // So is a data root only this machine mounts.
+    writeFileSync(lockPath, JSON.stringify(persistedRecord()))
+    expect(acquireOrcadInstanceLock(root, hooks()).record.machineIdentity).toBe('machine-a')
+  })
+
+  it('admits one owner when reclaimers of the same stale lock interleave', () => {
+    const root = makeRoot()
+    const lockPath = join(root, ORCAD_LOCK_FILE_NAME)
+    writeFileSync(lockPath, JSON.stringify(persistedRecord()))
+    const owners: { record: OrcadLockRecord }[] = []
+    const failures: unknown[] = []
+    const attempt = (lockHooks: OrcadInstanceLockHooks): void => {
+      try {
+        owners.push(acquireOrcadInstanceLock(root, lockHooks))
+      } catch (error) {
+        failures.push(error)
+      }
+    }
+    let raced = false
+    // A has read the stale record when B reclaims it and publishes its own; then, the moment
+    // anything renames B's live lock away, C tries the momentarily vacant path.
+    attempt(
+      hooks({
+        processIsAlive: () => {
+          if (!raced) {
+            raced = true
+            attempt(hooks())
+            afterLockRename.next = () => attempt(hooks())
+          }
+          return false
+        }
+      })
+    )
+
+    expect(owners).toHaveLength(1)
+    expect(readOrcadInstanceLockRecord(lockPath)?.nonce).toBe(owners[0]?.record.nonce)
+    expect(failures).toEqual([expect.objectContaining({ code: 'orcad_instance_lock_held' })])
+    expect(readdirSync(root)).toEqual([ORCAD_LOCK_FILE_NAME])
+  })
+
+  it('takes a new recovery generation only once every earlier owner provably exited', () => {
+    const root = makeRoot()
+    const lockPath = join(root, ORCAD_LOCK_FILE_NAME)
+    const stale = JSON.stringify(persistedRecord({ machineIdentity: 'machine-a' }))
+    const generation = (n: number): string => `${orcadLockRecoveryPrefix(lockPath, stale)}${n}`
+    const leaveRecovery = (n: number, owner: string | null): void => {
+      mkdirSync(generation(n))
+      if (owner !== null) {
+        writeFileSync(join(generation(n), 'owner'), owner)
+      }
+    }
+    const reclaimer = (overrides: Partial<OrcadLockRecord>): string =>
+      JSON.stringify(persistedRecord({ pid: 5150, nonce: 'reclaimer', ...overrides }))
+    const refused = expect.objectContaining({ message: expect.stringContaining('is reclaiming') })
+    // Only pid 5150, a reclaimer paused however long ago (stalled NFS, a suspended laptop), runs.
+    const paused = hooks({ processIsAlive: (pid) => pid === 5150 })
+    const reset = (): void => {
+      for (const entry of readdirSync(root)) {
+        rmSync(join(root, entry), { recursive: true, force: true })
+      }
+      writeFileSync(lockPath, stale)
+    }
+
+    for (const owner of [
+      reclaimer({ machineIdentity: 'machine-a' }),
+      // Dead here, but on another machine.
+      reclaimer({ pid: 6160, machineIdentity: 'machine-b' }),
+      'not a record'
+    ]) {
+      reset()
+      leaveRecovery(0, owner)
+      expect(() => acquireOrcadInstanceLock(root, paused)).toThrow(refused)
+      expect(readOrcadInstanceLockRecord(lockPath)?.nonce).toBe('stale')
+      expect(readFileSync(join(generation(0), 'owner'), 'utf8')).toBe(owner)
+    }
+    // An owner that cannot be read proves nothing either.
+    reset()
+    mkdirSync(join(generation(0), 'owner'), { recursive: true })
+    expect(() => acquireOrcadInstanceLock(root, paused)).toThrow(refused)
+    // Dead here, on a machine this cannot tell apart from this one.
+    reset()
+    leaveRecovery(0, reclaimer({ pid: 6160 }))
+    expect(() =>
+      acquireOrcadInstanceLock(root, hooks({ ...paused, dataRootIsOnLocalDisk: () => false }))
+    ).toThrow(refused)
+
+    // An owner that exited on this machine is a crash's; so is a generation a cleanup cut short.
+    for (const owner of [reclaimer({ pid: 6160, machineIdentity: 'machine-a' }), null]) {
+      reset()
+      leaveRecovery(0, owner)
+      const lock = acquireOrcadInstanceLock(root, paused)
+      expect(readOrcadInstanceLockRecord(lockPath)?.nonce).toBe(lock.record.nonce)
+      expect(readdirSync(root)).toEqual([ORCAD_LOCK_FILE_NAME])
+      lock.release()
+    }
+  })
+
+  it('never reuses a generation a failed recovery let go of', () => {
+    const root = makeRoot()
+    const lockPath = join(root, ORCAD_LOCK_FILE_NAME)
+    const stale = JSON.stringify(persistedRecord({ machineIdentity: 'machine-a' }))
+    const generation = (n: number): string => `${orcadLockRecoveryPrefix(lockPath, stale)}${n}`
+    writeFileSync(lockPath, stale)
+    // Its rename of the stale lock fails, so the stale record stays in place.
+    afterLockRename.failNext = true
+    expect(() => acquireOrcadInstanceLock(root, hooks())).toThrow(
+      expect.objectContaining({ code: 'orcad_instance_lock_held' })
+    )
+    expect(readdirSync(generation(0)).sort()).toEqual(['owner', 'released'])
+
+    // The next reclaimer, though this process is still alive, moves to a new generation.
+    const lock = acquireOrcadInstanceLock(
+      root,
+      hooks({ processIsAlive: (pid) => pid === process.pid })
+    )
+    expect(readOrcadInstanceLockRecord(lockPath)?.nonce).toBe(lock.record.nonce)
+    expect(readdirSync(root)).toEqual([ORCAD_LOCK_FILE_NAME])
+  })
+
+  it('never displaces a reclaimer that broke the same dead recovery owner first', () => {
+    const root = makeRoot()
+    const lockPath = join(root, ORCAD_LOCK_FILE_NAME)
+    const stale = JSON.stringify(persistedRecord({ machineIdentity: 'machine-a' }))
+    const generation = (n: number): string => `${orcadLockRecoveryPrefix(lockPath, stale)}${n}`
+    writeFileSync(lockPath, stale)
+    mkdirSync(generation(0))
+    const dead = JSON.stringify(persistedRecord({ pid: 6160, machineIdentity: 'machine-a' }))
+    writeFileSync(join(generation(0), 'owner'), dead)
+    const live = JSON.stringify(persistedRecord({ pid: 5150, machineIdentity: 'machine-a' }))
+    let raced = false
+    // B judges generation 0's owner dead at the same moment and takes generation 1 first.
+    const racing = hooks({
+      processIsAlive: (pid) => {
+        if (!raced && pid === 6160) {
+          raced = true
+          mkdirSync(generation(1))
+          writeFileSync(join(generation(1), 'owner'), live)
+        }
+        return pid === 5150
+      }
+    })
+    const refused = expect.objectContaining({ message: expect.stringContaining('is reclaiming') })
+    expect(() => acquireOrcadInstanceLock(root, racing)).toThrow(refused)
+    // And a third starter defers to B, the newest generation's live owner.
+    expect(() =>
+      acquireOrcadInstanceLock(root, hooks({ processIsAlive: (pid) => pid === 5150 }))
+    ).toThrow(refused)
+    expect(readFileSync(join(generation(1), 'owner'), 'utf8')).toBe(live)
+    expect(readFileSync(join(generation(0), 'owner'), 'utf8')).toBe(dead)
+    expect(readOrcadInstanceLockRecord(lockPath)?.nonce).toBe('stale')
+    expect(readdirSync(root)).toHaveLength(3)
+  })
+
   it.runIf(process.platform !== 'win32')(
     "leaves the desktop profile's permissions as they were",
     () => {
diff --git a/src/main/orcad/orcad-instance-lock.ts b/src/main/orcad/orcad-instance-lock.ts
index faa391ba116..72969972f3b 100644
--- a/src/main/orcad/orcad-instance-lock.ts
+++ b/src/main/orcad/orcad-instance-lock.ts
@@ -13,7 +13,7 @@
  * it says nothing about the daemon, which is what makes a non-destructive restart possible.
  */
 import { randomUUID } from 'node:crypto'
-import { linkSync, mkdirSync, renameSync, statSync, unlinkSync, writeFileSync } from 'node:fs'
+import { linkSync, mkdirSync, statSync, unlinkSync, writeFileSync } from 'node:fs'
 import { userInfo } from 'node:os'
 import { join } from 'node:path'
 import process from 'node:process'
@@ -31,11 +31,18 @@ import {
   OrcadInstanceLockError,
   type OrcadDataRootPrivacyHooks
 } from './orcad-data-root-privacy'
-import { readNodeFileSyncWithinLimit } from '../../shared/node-bounded-file-reader'
 import { hasErrorCode, isProcessAlive } from '../daemon/daemon-process-inspection'
+import { profileStateAccessMachineIdentity } from '../persistence/profile-state/profile-state-access-identity'
+import { orcadDataRootIsOnLocalDisk } from './orcad-data-root-locality'
+import {
+  MAX_ORCAD_LOCK_BYTES,
+  readBoundedLockFile,
+  reclaimAndPublish,
+  unlinkQuietly,
+  type OrcadLockRecoveryOwner
+} from './orcad-instance-lock-reclaim'
 
 export const ORCAD_LOCK_FILE_NAME = 'orcad.lock'
-const MAX_ORCAD_LOCK_BYTES = 64 * 1024
 // A writer finishes in milliseconds; a garbled record older than this has no live writer.
 const GARBLED_LOCK_GRACE_MS = 10_000
 
@@ -56,7 +63,12 @@ const OrcadLockRecordSchema = z.object({
   /** Distinguishes our record from a replacement written after we lost the race. */
   nonce: z.string().min(1).max(255),
   /** Absent in records orcad wrote before the desktop app shared this lock. */
-  role: z.enum(['orcad', 'desktop']).optional()
+  role: z.enum(['orcad', 'desktop']).optional(),
+  /**
+   * The holder's machine (as profile-state owners record it). Absent from older writers. A PID is
+   * evidence only on the machine that issued it, and a shared home can put two on one data root.
+   */
+  machineIdentity: z.string().min(1).max(255).optional().catch(undefined)
 })
 
 export type OrcadLockRecord = z.infer<typeof OrcadLockRecordSchema>
@@ -89,6 +101,9 @@ export type OrcadInstanceLockHooks = OrcadDataRootPrivacyHooks & {
    * hold Electron's single-instance lock first, which is what lets it reclaim a desktop record.
    */
   role?: 'orcad' | 'desktop'
+  machineIdentity?: () => string | null
+  /** Whether no other machine can reach the data root, for records that name no machine. */
+  dataRootIsOnLocalDisk?: (dataRoot: string) => boolean
 }
 
 function defaultIdentity(): string {
@@ -147,6 +162,7 @@ export function acquireOrcadInstanceLock(
   }
 
   const lockPath = join(dataRoot, ORCAD_LOCK_FILE_NAME)
+  const machineIdentity = (hooks.machineIdentity ?? profileStateAccessMachineIdentity)()
   const record: OrcadLockRecord = {
     pid: process.pid,
     startedAtMs: readStartedAt(process.pid),
@@ -155,7 +171,8 @@ export function acquireOrcadInstanceLock(
     version: (hooks.version ?? (() => process.env.ORCA_VERSION ?? 'unknown'))(),
     acquiredAt: (hooks.now ?? (() => new Date()))().toISOString(),
     nonce: randomUUID(),
-    role: hooks.role ?? 'orcad'
+    role: hooks.role ?? 'orcad',
+    ...(machineIdentity ? { machineIdentity } : {})
   }
   const serialized = JSON.stringify(record)
 
@@ -183,6 +200,22 @@ export function acquireOrcadInstanceLock(
     return makeLock(lockPath, record)
   }
 
+  const dataRootIsLocal = (): boolean =>
+    (hooks.dataRootIsOnLocalDisk ?? orcadDataRootIsOnLocalDisk)(dataRoot)
+  // A recovery lock is ours by the same record, and broken only on the same proof of exit.
+  const recoveryOwner: OrcadLockRecoveryOwner = {
+    record: serialized,
+    exited: (ownerRecord) => {
+      const owner = parseOrcadInstanceLockRecord(ownerRecord)
+      return Boolean(
+        owner &&
+        owner.identity === identity &&
+        holderLocality(owner, record, dataRootIsLocal) === 'local' &&
+        (!isAlive(owner.pid) || holderIdentity(owner) === 'differs')
+      )
+    }
+  }
+
   const existingContents = readBoundedLockFile(lockPath)
   if (existingContents === null) {
     // Why fail closed: a record we cannot read proves nothing about its holder having exited.
@@ -198,7 +231,8 @@ export function acquireOrcadInstanceLock(
     // Only a torn write (an older build, or a crash mid-write) leaves this; once it has aged
     // past any live writer, no owner can be behind it.
     if (garbledLockIsAbandoned(lockPath)) {
-      return reclaimAndPublish(lockPath, dataRoot, existingContents, publish, record)
+      reclaimAndPublish(lockPath, dataRoot, existingContents, publish, recoveryOwner)
+      return makeLock(lockPath, record)
     }
     throw new OrcadInstanceLockError(
       'orcad_instance_lock_held',
@@ -213,6 +247,18 @@ export function acquireOrcadInstanceLock(
         'root corrupts it. Give each its own ORCA_USER_DATA.'
     )
   }
+  const locality = holderLocality(existing, record, dataRootIsLocal)
+  if (locality !== 'local') {
+    // Its PID means nothing here, so neither a missing nor a reused one proves it exited.
+    throw new OrcadInstanceLockError(
+      'orcad_instance_lock_held',
+      `${describeLockHolder(existing)} (pid ${existing.pid}, started ${existing.acquiredAt}) ` +
+        `holds the data root ${dataRoot}${locality === 'foreign' ? ' from another machine' : ''}. ` +
+        'Orca cannot check from here whether it is still running, because this data root may be ' +
+        `shared with other machines. Stop it there, or if it is not running anywhere, delete ` +
+        `${lockPath} and try again.`
+    )
+  }
   // The desktop takes this lock only after Electron's single-instance lock, which already proves
   // no other desktop runs; a desktop record is then stale even when its PID was reused.
   const staleDesktopRecord = hooks.role === 'desktop' && existing.role === 'desktop'
@@ -228,7 +274,26 @@ export function acquireOrcadInstanceLock(
         'before starting another, or use a different ORCA_USER_DATA.'
     )
   }
-  return reclaimAndPublish(lockPath, dataRoot, existingContents, publish, record)
+  reclaimAndPublish(lockPath, dataRoot, existingContents, publish, recoveryOwner)
+  return makeLock(lockPath, record)
+}
+
+/**
+ * Same machine: the same machine identity, or (records without one) the same kernel boot. A record
+ * that names neither is local only when no other machine can reach the data root.
+ */
+function holderLocality(
+  holder: OrcadLockRecord,
+  self: OrcadLockRecord,
+  dataRootIsOnLocalDisk: () => boolean
+): 'local' | 'foreign' | 'unknown' {
+  if (holder.machineIdentity && self.machineIdentity) {
+    return holder.machineIdentity === self.machineIdentity ? 'local' : 'foreign'
+  }
+  if (holder.bootId && holder.bootId === self.bootId) {
+    return 'local'
+  }
+  return dataRootIsOnLocalDisk() ? 'local' : 'unknown'
 }
 
 /** A record without a boot id cannot tell its holder from a reused PID; say so. */
@@ -250,50 +315,6 @@ function possiblyReusedPidError(
   )
 }
 
-/**
- * Why rename-and-then-publish rather than unlink-and-write: rename claims one exact directory
- * entry, so a replacement written between our read and our write stays at the canonical path
- * and wins — we never delete a record we did not inspect.
- */
-function reclaimAndPublish(
-  lockPath: string,
-  dataRoot: string,
-  inspectedContents: string,
-  publish: () => boolean,
-  record: OrcadLockRecord
-): OrcadInstanceLock {
-  const claimPath = `${lockPath}.stale-${process.pid}-${randomUUID()}`
-  try {
-    renameSync(lockPath, claimPath)
-  } catch {
-    throw new OrcadInstanceLockError(
-      'orcad_instance_lock_held',
-      `Could not reclaim the stale orcad instance lock at ${lockPath}; another process is ` +
-        'holding it. Retry, or stop the other orcad.'
-    )
-  }
-  // A contender may have replaced the entry after the liveness check; never displace its record.
-  const claimedContents = readBoundedLockFile(claimPath)
-  if (claimedContents !== inspectedContents) {
-    restoreDisplacedLock(claimPath, lockPath, claimedContents)
-    throw new OrcadInstanceLockError(
-      'orcad_instance_lock_held',
-      `The orcad instance lock at ${lockPath} changed while reclaiming a stale record.`
-    )
-  }
-  const published = publish()
-  // A uniquely named claim is inert either way.
-  unlinkQuietly(claimPath)
-  if (!published) {
-    // Someone else claimed it first. Their record is authoritative; ours is not.
-    throw new OrcadInstanceLockError(
-      'orcad_instance_lock_held',
-      `Another orcad took the data root ${dataRoot} while this one was reclaiming a stale lock.`
-    )
-  }
-  return makeLock(lockPath, record)
-}
-
 function garbledLockIsAbandoned(lockPath: string): boolean {
   try {
     return Date.now() - statSync(lockPath).mtimeMs > GARBLED_LOCK_GRACE_MS
@@ -302,49 +323,10 @@ function garbledLockIsAbandoned(lockPath: string): boolean {
   }
 }
 
-function unlinkQuietly(path: string): void {
-  try {
-    unlinkSync(path)
-  } catch {
-    // Already gone, or inert under its unique name.
-  }
-}
-
 function describeLockHolder(record: OrcadLockRecord): string {
   return record.role === 'desktop' ? 'The Orca desktop app' : 'Another orcad'
 }
 
-/** No-clobber restore: a third contender's newer record at the canonical path stays authoritative. */
-function restoreDisplacedLock(
-  claimPath: string,
-  lockPath: string,
-  claimedContents: string | null
-): void {
-  try {
-    linkSync(claimPath, lockPath)
-    unlinkSync(claimPath)
-  } catch {
-    if (claimedContents === null) {
-      return
-    }
-    try {
-      writeFileSync(lockPath, claimedContents, { flag: 'wx', mode: 0o600 })
-      unlinkSync(claimPath)
-    } catch {
-      // A newer contender won, or restoration is unavailable; fail closed.
-    }
-  }
-}
-
-function readBoundedLockFile(path: string): string | null {
-  try {
-    const { buffer, stats } = readNodeFileSyncWithinLimit(path, MAX_ORCAD_LOCK_BYTES)
-    return stats.isFile() ? buffer.toString('utf8') : null
-  } catch {
-    return null
-  }
-}
-
 function makeLock(lockPath: string, record: OrcadLockRecord): OrcadInstanceLock {
   let released = false
   return {
diff --git a/src/main/ssh/orcad-machine-claim-command.ts b/src/main/ssh/orcad-machine-claim-command.ts
new file mode 100644
index 00000000000..73c69fbe230
--- /dev/null
+++ b/src/main/ssh/orcad-machine-claim-command.ts
@@ -0,0 +1,55 @@
+import { randomUUID } from 'node:crypto'
+import { shellEscape } from './ssh-connection-utils'
+import { RELAY_REMOTE_DIR } from './relay-protocol'
+import {
+  ORCAD_MACHINE_CLAIM_FILENAME,
+  ORCAD_MACHINE_CLAIM_SCHEMA_VERSION
+} from './orcad-machine-home'
+import { joinRemotePath, type RemoteHostPlatform } from './ssh-remote-platform'
+
+/** Under `~/.orca-remote`: decides the claim when hard links cannot (see the claim command). */
+export const ORCAD_MACHINE_CLAIM_ARBITER_DIRNAME = 'orcad-machine.arbiter'
+
+/**
+ * Publishes the claim exclusively and prints whichever claim won. Why a hard link: `ln` is atomic
+ * and exclusive on NFS too, and a reader never sees a half-written claim. Where `ln` cannot decide
+ * (no hard links, or an empty claim a full disk left), a directory rename picks the winner: it is
+ * atomic everywhere, and the arbiter arrives already holding the winner's claim, so a crash leaves
+ * no half-taken lock. Every machine then republishes that same claim, so concurrent writes agree.
+ */
+export function orcadMachineClaimCommand(
+  host: RemoteHostPlatform,
+  loginHome: string,
+  machineKey: string
+): string {
+  const remoteDir = (...parts: string[]): string =>
+    shellEscape(joinRemotePath(host, loginHome, RELAY_REMOTE_DIR, ...parts))
+  const attempt = randomUUID()
+  const base = remoteDir()
+  const claim = remoteDir(ORCAD_MACHINE_CLAIM_FILENAME)
+  const partial = remoteDir(`${ORCAD_MACHINE_CLAIM_FILENAME}.partial.${attempt}`)
+  const stageName = `${ORCAD_MACHINE_CLAIM_ARBITER_DIRNAME}.partial.${attempt}`
+  const stage = remoteDir(stageName)
+  const stageClaim = remoteDir(stageName, ORCAD_MACHINE_CLAIM_FILENAME)
+  const arbiter = remoteDir(ORCAD_MACHINE_CLAIM_ARBITER_DIRNAME)
+  const arbiterClaim = remoteDir(ORCAD_MACHINE_CLAIM_ARBITER_DIRNAME, ORCAD_MACHINE_CLAIM_FILENAME)
+  // `mv` into an existing arbiter nests the stage inside it instead of replacing it.
+  const nestedStage = remoteDir(ORCAD_MACHINE_CLAIM_ARBITER_DIRNAME, stageName)
+  const body = shellEscape(
+    JSON.stringify({
+      schemaVersion: ORCAD_MACHINE_CLAIM_SCHEMA_VERSION,
+      machineKey
+    })
+  )
+  return [
+    `umask 077; mkdir -p ${base} || exit 65;`,
+    `printf %s ${body} > ${partial} || exit 65;`,
+    // Once an arbiter exists it decides any claim that later goes missing or empty.
+    `if { [ -d ${arbiter} ] || ! ln ${partial} ${claim} 2>/dev/null; } && [ ! -s ${claim} ]; then`,
+    `{ mkdir ${stage} && printf %s ${body} > ${stageClaim}; } || { rm -rf ${stage} ${partial}; exit 65; };`,
+    `mv ${stage} ${arbiter} 2>/dev/null; rm -rf ${stage} ${nestedStage};`,
+    `[ -s ${arbiterClaim} ] && cat ${arbiterClaim} > ${partial} || { rm -f ${partial}; exit 65; };`,
+    `[ -s ${claim} ] || mv -f ${partial} ${claim};`,
+    `fi; rm -f ${partial}; cat ${claim}`
+  ].join(' ')
+}
diff --git a/src/main/ssh/orcad-machine-home.test.ts b/src/main/ssh/orcad-machine-home.test.ts
index 5c39b50e0d1..f7eaf2fb96a 100644
--- a/src/main/ssh/orcad-machine-home.test.ts
+++ b/src/main/ssh/orcad-machine-home.test.ts
@@ -46,10 +46,10 @@ const {
   orcadMachineIdentity,
   orcadMachineOwnershipUnresolvedMessage,
   parseOrcadMachineHomeProbe,
-  ORCAD_MACHINE_CLAIM_ARBITER_DIRNAME,
   ORCAD_MACHINE_CLAIM_FILENAME,
   ORCAD_MACHINE_HOMES_DIRNAME
 } = await import('./orcad-machine-home')
+const { ORCAD_MACHINE_CLAIM_ARBITER_DIRNAME } = await import('./orcad-machine-claim-command')
 const { computeRemoteInstallDir } = await import('./remote-versioned-install')
 const { ORCAD_INSTALL_MODEL } = await import('./remote-install-model')
 const { orcadLivenessProbeCommand, ORCAD_PID_FILENAME } = await import('./orcad-remote-launch')
@@ -263,6 +263,28 @@ describe.runIf(POSIX)('resolveOrcadMachineHome on a home several machines share'
     expect(await resolveOrcadMachineHome(a, true)).toEqual(loginHomeResult())
   })
 
+  it('never lets another Mac claim a live install whose owner recorded no machine identity', async () => {
+    const nfs = { homeSource: 'filer:/export/home' }
+    const b = machine('node-b', 'node-b', nfs)
+    mkdirSync(join(home, '.orca-remote', 'orcad-0.1.0+abc'), { recursive: true })
+    writeFileSync(join(home, '.orca-remote', 'orcad-active.json'), '{"schemaVersion":1}')
+    // node-a's older orcad is running: its owner names only node-a's hostname.
+    legacyOwner('node-a', null)
+
+    await expect(resolveOrcadMachineHome(b, true)).rejects.toThrow(
+      /recorded only a hostname[\s\S]*If node-b set it up/
+    )
+    expect(existsSync(claimPath())).toBe(false)
+    // The same Mac under a drifted hostname is just as unprovable, so it is not claimed either.
+    await expect(
+      resolveOrcadMachineHome(machine('node-a', 'cafe-wifi', nfs), true)
+    ).rejects.toThrow(/recorded only a hostname/)
+    // node-a, still under the name its owner recorded, keeps its own install.
+    expect(await resolveOrcadMachineHome(machine('node-a', 'node-a', nfs), true)).toEqual(
+      loginHomeResult()
+    )
+  })
+
   it('lets exactly one machine claim a home on a filesystem without hard links', async () => {
     const a = machine('node-a', 'node-a', { noHardLinks: true, claimWriteDelay: 0 })
     const b = machine('node-b', 'node-b', { noHardLinks: true, claimWriteDelay: 0.5 })
@@ -443,6 +465,24 @@ describe('decideOrcadMachineHome', () => {
     expect(decideOrcadMachineHome(unrecorded, linux).kind).toBe('machine-home')
   })
 
+  it('claims no install whose owner names neither its machine nor a matching hostname', () => {
+    const uuid = '12345678-1234-1234-1234-123456789abc'
+    const mac = orcadMachineIdentity(darwin, `"IOPlatformUUID" = "${uuid}"`, 'mac-b')
+    const otherMac = { host: 'mac-a', machineIdentity: null }
+    expect(
+      decideOrcadMachineHome(probe(darwin, { identity: mac, dataRootOwners: [otherMac] }), darwin)
+    ).toEqual({ kind: 'unresolved', machineKey: mac?.key, cause: 'unattributed-owner' })
+    // A local disk cannot be another machine's.
+    expect(
+      decideOrcadMachineHome(
+        probe(darwin, { identity: mac, dataRootOwners: [otherMac], homeOnLocalDisk: true }),
+        darwin
+      ).kind
+    ).toBe('claim')
+    const nameless = probe(linux, { dataRootOwners: [{ host: null, machineIdentity: null }] })
+    expect(decideOrcadMachineHome(nameless, linux).kind).toBe('unresolved')
+  })
+
   it('reads a Linux home’s filesystem type before its df source', () => {
     const output = (installed: string, type: string, source: string): string =>
       [
diff --git a/src/main/ssh/orcad-machine-home.ts b/src/main/ssh/orcad-machine-home.ts
index c94e1692d3d..b22b47f47a2 100644
--- a/src/main/ssh/orcad-machine-home.ts
+++ b/src/main/ssh/orcad-machine-home.ts
@@ -10,7 +10,7 @@
  * immutable install stays per machine: the runtime store's GC judges "in use" from processes it
  * can see, so a store shared across machines could be collected under another machine's server.
  */
-import { createHash, randomUUID } from 'node:crypto'
+import { createHash } from 'node:crypto'
 import { shellEscape } from './ssh-connection-utils'
 import { RELAY_REMOTE_DIR } from './relay-protocol'
 import { ORCAD_ACTIVATION_FILENAME } from './orcad-activation-record'
@@ -27,8 +27,6 @@ export const ORCAD_MACHINE_CLAIM_FILENAME = 'orcad-machine.json'
 /** Under `~/.orca-remote`: one orcad home per other machine sharing this `$HOME`. */
 export const ORCAD_MACHINE_HOMES_DIRNAME = 'hosts'
 export const ORCAD_MACHINE_CLAIM_SCHEMA_VERSION = 1
-/** Under `~/.orca-remote`: decides the claim when hard links cannot (see the claim command). */
-export const ORCAD_MACHINE_CLAIM_ARBITER_DIRNAME = 'orcad-machine.arbiter'
 
 const SECTION = '__ORCA_MACHINE_HOME_SECTION__'
 const CLAIM_ABSENT = 'ABSENT'
@@ -92,8 +90,11 @@ export type OrcadMachineHomeDecision =
   | { kind: 'login-home' }
   | { kind: 'claim'; machineKey: string }
   | { kind: 'machine-home'; machineKey: string }
-  /** A stopped pre-claim install on a shared home: no record says which machine ran it. */
-  | { kind: 'unresolved'; machineKey: string }
+  /** A pre-claim install on a shared home that no record attributes to a machine. */
+  | { kind: 'unresolved'; machineKey: string; cause: OrcadMachineOwnershipUnresolvedCause }
+
+/** A stopped install left no owner; an older orcad's owner recorded no machine identity. */
+export type OrcadMachineOwnershipUnresolvedCause = 'stopped-install' | 'unattributed-owner'
 
 /**
  * Reads the login home, this machine's identity, the claim, and who holds the shared data root,
@@ -242,20 +243,24 @@ function parseOwnerLine(line: string): OrcadMachineHomeProbe['dataRootOwners'][n
   }
 }
 
-/** True when a data-root owner names a machine other than this one. */
-function isForeignOwner(
+/** Whether a data-root owner is this machine, another one, or cannot be told apart. */
+function ownerMachine(
   owner: OrcadMachineHomeProbe['dataRootOwners'][number],
   identity: OrcadMachineIdentity,
   host: RemoteHostPlatform
-): boolean {
+): 'this' | 'foreign' | 'unknown' {
   if (owner.machineIdentity) {
-    return owner.machineIdentity.toLowerCase() !== identity.machineIdentity
+    return owner.machineIdentity.toLowerCase() === identity.machineIdentity ? 'this' : 'foreign'
   }
-  // An owner too old to record its machine is judged by hostname, except on drifting macOS names.
-  return (
-    host.os !== 'darwin' &&
-    Boolean(owner.host && identity.hostname && owner.host !== identity.hostname)
-  )
+  // An owner too old to record its machine is judged by hostname.
+  if (!owner.host || !identity.hostname) {
+    return 'unknown'
+  }
+  if (owner.host === identity.hostname) {
+    return 'this'
+  }
+  // A macOS hostname follows the network, so a different one may still be this Mac.
+  return host.os === 'darwin' ? 'unknown' : 'foreign'
 }
 
 /**
@@ -263,8 +268,9 @@ function isForeignOwner(
  * data root: that one is the install's owner, and claiming it here would judge its PIDs on the
  * wrong machine. A machine that already has its own home keeps it, so a later claim cannot move
  * it off a server still running there. An unreadable claim throws: guessing can strand a server.
- * A cleanly stopped install leaves no owner, so on a home that may be shared its machine is
- * unknown: claiming it could hand one machine's pairing and projects to another.
+ * A cleanly stopped install leaves no owner, and an older orcad's owner may name no machine
+ * (macOS hostnames drift), so on a home that may be shared its machine is unknown: claiming it
+ * could hand one machine's pairing, projects and instance lock to another.
  */
 export function decideOrcadMachineHome(
   probe: OrcadMachineHomeProbe,
@@ -284,27 +290,37 @@ export function decideOrcadMachineHome(
       ? { kind: 'login-home' }
       : { kind: 'machine-home', machineKey: identity.key }
   }
-  if (
-    probe.machineHomeKeys.includes(identity.key) ||
-    probe.dataRootOwners.some((owner) => isForeignOwner(owner, identity, host))
-  ) {
+  const owners = probe.dataRootOwners.map((owner) => ownerMachine(owner, identity, host))
+  if (probe.machineHomeKeys.includes(identity.key) || owners.includes('foreign')) {
     return { kind: 'machine-home', machineKey: identity.key }
   }
-  return probe.loginHomeInstalled && probe.dataRootOwners.length === 0 && !probe.homeOnLocalDisk
-    ? { kind: 'unresolved', machineKey: identity.key }
+  if (probe.homeOnLocalDisk) {
+    return { kind: 'claim', machineKey: identity.key }
+  }
+  // Claiming another machine's install would judge its PIDs here and admit a second writer.
+  if (owners.includes('unknown')) {
+    return { kind: 'unresolved', machineKey: identity.key, cause: 'unattributed-owner' }
+  }
+  return probe.loginHomeInstalled && owners.length === 0
+    ? { kind: 'unresolved', machineKey: identity.key, cause: 'stopped-install' }
     : { kind: 'claim', machineKey: identity.key }
 }
 
 /** Names the one-line claim the user can run on the machine that set the server up. */
 export function orcadMachineOwnershipUnresolvedMessage(
   machineKey: string,
-  hostname: string | null
+  hostname: string | null,
+  cause: OrcadMachineOwnershipUnresolvedCause = 'stopped-install'
 ): string {
   const claim = JSON.stringify({ schemaVersion: ORCAD_MACHINE_CLAIM_SCHEMA_VERSION, machineKey })
   const path = `~/${RELAY_REMOTE_DIR}/${ORCAD_MACHINE_CLAIM_FILENAME}`
+  const why =
+    cause === 'stopped-install'
+      ? 'that stopped server left no record of its own'
+      : 'the older Orca server there recorded only a hostname, which does not identify its machine'
   return (
-    `Orca can't tell which machine set up the stopped Orca server in ~/${RELAY_REMOTE_DIR}: ` +
-    'this home may be shared by several machines, and that server left no record of its own. ' +
+    `Orca can't tell which machine set up the Orca server in ~/${RELAY_REMOTE_DIR}: ` +
+    `this home may be shared by several machines, and ${why}. ` +
     'If another machine set it up, connect to that one and run the command shown there. ' +
     `If ${hostname ?? 'this machine'} set it up, run this on it, then reconnect: ` +
     `mkdir -p ~/${RELAY_REMOTE_DIR} && printf '%s' '${claim}' > ${path}`
@@ -330,47 +346,3 @@ export function orcadMachineDataRoot(
 ): string {
   return joinRemotePath(host, loginHome, `.orca-${machineKey.slice(0, 8)}`)
 }
-
-/**
- * Publishes the claim exclusively and prints whichever claim won. Why a hard link: `ln` is atomic
- * and exclusive on NFS too, and a reader never sees a half-written claim. Where `ln` cannot decide
- * (no hard links, or an empty claim a full disk left), a directory rename picks the winner: it is
- * atomic everywhere, and the arbiter arrives already holding the winner's claim, so a crash leaves
- * no half-taken lock. Every machine then republishes that same claim, so concurrent writes agree.
- */
-export function orcadMachineClaimCommand(
-  host: RemoteHostPlatform,
-  loginHome: string,
-  machineKey: string
-): string {
-  const remoteDir = (...parts: string[]): string =>
-    shellEscape(joinRemotePath(host, loginHome, RELAY_REMOTE_DIR, ...parts))
-  const attempt = randomUUID()
-  const base = remoteDir()
-  const claim = remoteDir(ORCAD_MACHINE_CLAIM_FILENAME)
-  const partial = remoteDir(`${ORCAD_MACHINE_CLAIM_FILENAME}.partial.${attempt}`)
-  const stageName = `${ORCAD_MACHINE_CLAIM_ARBITER_DIRNAME}.partial.${attempt}`
-  const stage = remoteDir(stageName)
-  const stageClaim = remoteDir(stageName, ORCAD_MACHINE_CLAIM_FILENAME)
-  const arbiter = remoteDir(ORCAD_MACHINE_CLAIM_ARBITER_DIRNAME)
-  const arbiterClaim = remoteDir(ORCAD_MACHINE_CLAIM_ARBITER_DIRNAME, ORCAD_MACHINE_CLAIM_FILENAME)
-  // `mv` into an existing arbiter nests the stage inside it instead of replacing it.
-  const nestedStage = remoteDir(ORCAD_MACHINE_CLAIM_ARBITER_DIRNAME, stageName)
-  const body = shellEscape(
-    JSON.stringify({
-      schemaVersion: ORCAD_MACHINE_CLAIM_SCHEMA_VERSION,
-      machineKey
-    })
-  )
-  return [
-    `umask 077; mkdir -p ${base} || exit 65;`,
-    `printf %s ${body} > ${partial} || exit 65;`,
-    // Once an arbiter exists it decides any claim that later goes missing or empty.
-    `if { [ -d ${arbiter} ] || ! ln ${partial} ${claim} 2>/dev/null; } && [ ! -s ${claim} ]; then`,
-    `{ mkdir ${stage} && printf %s ${body} > ${stageClaim}; } || { rm -rf ${stage} ${partial}; exit 65; };`,
-    `mv ${stage} ${arbiter} 2>/dev/null; rm -rf ${stage} ${nestedStage};`,
-    `[ -s ${arbiterClaim} ] && cat ${arbiterClaim} > ${partial} || { rm -f ${partial}; exit 65; };`,
-    `[ -s ${claim} ] || mv -f ${partial} ${claim};`,
-    `fi; rm -f ${partial}; cat ${claim}`
-  ].join(' ')
-}
diff --git a/src/main/ssh/orcad-remote-context.ts b/src/main/ssh/orcad-remote-context.ts
index 5047f2140f1..20ed0908965 100644
--- a/src/main/ssh/orcad-remote-context.ts
+++ b/src/main/ssh/orcad-remote-context.ts
@@ -12,7 +12,6 @@ import {
 } from './orcad-host-transport'
 import {
   decideOrcadMachineHome,
-  orcadMachineClaimCommand,
   orcadMachineDataRoot,
   orcadMachineHomeDir,
   orcadMachineHomeProbeCommand,
@@ -20,6 +19,7 @@ import {
   parseOrcadMachineClaim,
   parseOrcadMachineHomeProbe
 } from './orcad-machine-home'
+import { orcadMachineClaimCommand } from './orcad-machine-claim-command'
 import { execOrcadRemote, type OrcadRemoteExecTarget } from './orcad-remote-runtime-control'
 import { readRemoteHomeCommand } from './ssh-remote-commands'
 import { shellEscape } from './ssh-connection-utils'
@@ -130,7 +130,11 @@ export async function resolveOrcadMachineHome(
   let decision = decideOrcadMachineHome(probe, host)
   if (decision.kind === 'unresolved') {
     throw new Error(
-      orcadMachineOwnershipUnresolvedMessage(decision.machineKey, probe.identity?.hostname ?? null)
+      orcadMachineOwnershipUnresolvedMessage(
+        decision.machineKey,
+        probe.identity?.hostname ?? null,
+        decision.cause
+      )
     )
   }
   if (decision.kind === 'claim') {