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 confirmedSymptom
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
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>.sparsebundlefor 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
.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
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
.sparsebundlebackups for Macs that are no longer in use
.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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