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Time Machine Backup Failures to Synology NAS

Pieter Engelbrecht
  • Updated

Time Machine Backup Failures to Synology NAS

Section: Backup & Recovery Tags: time-machine, synology, nas, backup-failure, sparse-bundle, afp, dsm Source tickets: #17923, #19037 Confidence: High — recurring pattern across multiple clients, root cause identified and confirmed

Symptom

One or more Macs display "Time Machine — Latest Backup: Failed" in the menu bar. Clicking "Back Up Now" fails immediately. The Synology NAS is reachable on the network and other file sharing works normally.

Common variants:

  • "Backup disk is in use" error
  • Backup appeared to complete but progress bar resets
  • Multiple machines suddenly failing at the same time (suggests NAS-side stuck connection)

Root Cause

Most common: A prior backup session terminated uncleanly (Mac slept, network dropped, NAS rebooted). The AFP/SMB session on the Synology was not released, leaving the sparse bundle "locked". Time Machine sees the disk as in use and refuses to start a new session. Secondary cause: The sparse bundle on the NAS has grown corrupt, or a user account on the Synology used for Time Machine backups has lost its password match (after a DSM update or password policy change). Less common: The volume or share has run out of space and Time Machine cannot allocate its next backup chunk.

Prerequisites

  • Admin access to Synology DSM (web UI at the NAS IP, default port 5000)
  • TeamViewer or Screen Share to the affected Mac(s)
  • Know the Time Machine share name and the user account used for TM auth on the NAS

Remediation Steps

Step 1 — Kill stuck sessions on the Synology

  • Log in to Synology DSM (browser → http://<NAS-IP>:5000)
  • Go to Control Panel → File Services → Connected Users (or in newer DSM: File Station → Tools → Connected Users)
  • Look for any sessions connected to the Time Machine share — especially sessions showing the username used for TM backups
  • Click the red X / Disconnect icon to kill the session
  • On the affected Mac: open System Preferences → Time Machine → click Back Up Now
Expected result: Backup starts and completes normally. Risk: None — killing an idle AFP/SMB session does not affect data.

Step 2 — Reset the sparse bundle (if Step 1 fails)

If the sparse bundle itself is corrupt or flagged as in-use:

  • In DSM File Station, navigate to the Time Machine share
  • Find the <MacName>.sparsebundle for the failing Mac
  • Rename it to <MacName>.sparsebundle.OLD (do not delete yet — keep as rollback)
  • On the Mac: System Preferences → Time Machine → Select Backup Disk — re-select the same Synology share
  • Time Machine will create a new sparse bundle and run a full fresh backup
Expected result: First backup after reset will be a full backup (slow — hours). Subsequent backups will be incremental. Risk: The previous backup history for this Mac is abandoned. The old .OLD bundle can be deleted once the new backup completes successfully (confirm with client before deleting).

Step 3 — Re-create the Time Machine user account on the Synology

If the password or account on the NAS no longer matches:

  • In DSM: Control Panel → User & Group — find the user account used for Time Machine (often tm_<machinename> or a shared account)
  • Reset the password to something known
  • On the Mac: in Keychain Access, search for the Synology hostname — delete the old entry
  • Go to System Preferences → Time Machine → Select Disk — re-connect and enter the new credentials
  • Test backup
Expected result: Mac authenticates cleanly and backup runs. Risk: None.

Step 4 — Check NAS volume free space

  • In DSM: Storage Manager → Volume — check available space
  • Time Machine needs at least 1–2× the Mac's used storage free to operate
  • If full: in the Time Machine share, delete the oldest .sparsebundle backups for Macs that are no longer in use
Expected result: Sufficient space available and backup proceeds. Risk: Caution — confirm which .sparsebundle files belong to which machine before deleting. Client confirmation required.

Verification

After fix, open Time Machine preferences and confirm "Latest Backup: [today's date and time]". Check at least two consecutive backups complete successfully before closing the ticket.

If This Fails

If the same machines fail repeatedly on a recurring basis (weekly or after each NAS reboot), the pattern is likely DSM's AFP stack losing mount registrations. Consider:

  • Upgrading DSM to the latest version
  • Switching the Time Machine protocol from AFP to SMB (available in DSM 6.2+): Control Panel → File Services → SMB → Enable Time Machine over SMB
  • For large deployments (4+ Macs), evaluate iSCSI target on the NAS instead of AFP share — more reliable for concurrent multi-Mac Time Machine

Related

  • Article: New Synology NAS Setup — for initial Time Machine share configuration
  • Article: Synology CloudStation Sync File Limit — separate NAS issue, not backup-related

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