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What to do if Time Machine backup fails?

Applicable Models​

All NAS models that support SMB backup

Symptoms​

  • After setting the NAS as the Time Machine backup destination on your Mac, the backup cannot proceed normally.
  • The Mac system indicates that the backup disk is full and the backup cannot be completed.
  • After checking the NAS, the total storage space is not completely used up, but the backup still fails.

Causes​

This issue is usually caused by one of the following reasons:

  • Insufficient total capacity or user quota on the backup disk.
  • NAS snapshots or Recycle Bin functions occupy space that should be freed by deleted backup files.
  • Improper SMB protocol configuration, leading to poor compatibility with Time Machine.
  • Insufficient free space on the Mac local startup disk.
  • Software issues with either the macOS or the NAS system.

Follow the steps below to troubleshoot.

Device Troubleshooting:​

1. Check Backup Space and Quotas​

  • Verify sufficient physical space: For Time Machine to work properly, the Volume containing the shared folder on the NAS should be at least 2 to 3 times the size of your Mac's internal hard drive. For example, for a 512GB Mac, it is recommended to reserve at least 1TB of space.
  • Check shared folder storage quota: Go to Control Panel > Shared Folders, check the "Storage Quota" set for the authorized directory (shared folder), and verify the used capacity via file management. If the used capacity reaches the quota limit, the backup will fail even if there is overall space on the NAS. Increase or remove the storage quota limit as appropriate.
  • Try creating a new shared folder and set it as the Time Machine backup authorized directory.

2. Address NAS Space Reclamation Mechanisms (Common Cause)​

NAS file system snapshots and Recycle Bin may lock space that has been deleted by Time Machine, preventing macOS from seeing the "available space" as actually freed.

  • Check and delete old snapshots:
    • Go to the Backup Center in the NAS management interface, locate file system snapshots, and manually delete older snapshots to release locked space.
    • If the Snapshot application is installed, also check whether file snapshots have been enabled for the Time Machine authorized directory and delete old snapshots.
  • Empty and disable Recycle Bin: Confirm that the Recycle Bin for the backup folder is disabled. If it is enabled, empty it first, then disable the feature to avoid conflicts.

3. Check and Optimize SMB Protocol Configuration​

Time Machine has specific requirements for advanced SMB protocol features.

  • Enable necessary options: Ensure that the NAS SMB service has "Opportunistic Locking", "Leases", and other key options enabled.
  • Set protocol version: It is recommended to set the maximum SMB protocol version to SMB 3.
  • Confirm Bonjour broadcast: Ensure that the NAS is correctly broadcasting its Time Machine service via Bonjour so that the Mac can discover and connect properly.

4. Check Mac Local Status​

  • Free up local storage space
    Ensure the Mac startup disk has at least 20% free space (at least 40 GB is recommended) for the local snapshot mechanism to work properly. If space is low, refer to Free up storage space on your Mac and try the backup again. If the issue persists, free up more space.
  • Re-add the backup disk
    Open Time Machine settings, remove the currently assigned backup disk. Then, in Finder, reconnect to the backup shared folder using smb://your_NAS_address. Finally, go back to Time Machine settings and re-select the disk as the backup target.
  • Update system software
    Check for and install the latest available macOS updates. Bugs in certain system versions can interfere with the backup process; updating the system often resolves such issues.

Collect Logs and Contact TerraMaster Support​

If Time Machine backup still fails after completing all the checks on the NAS and Mac, please collect the following information and contact TerraMaster support.

Information to Collect​

  • macOS version
  • NAS model
  • Time Machine backup configuration
  • The error message displayed when backup fails
  • Confirm whether the Mac is properly connected to the file service used for backup
  • Screenshot of the error message shown by Time Machine

Steps to Collect Time Machine Logs​

  1. Open Terminal on your Mac.
  2. Enter the following command to start logging:
log stream --level debug --predicate 'subsystem == "com.apple.TimeMachine"' --style compact &> ~/Desktop/tm_debug.log
  1. Run the Time Machine backup again.
  2. After the backup fails, return to the terminal and press "Ctrl" + "C" to stop collecting logs.
  3. Enter the following command:
ps -ef > ~/Desktop/ps.log
  1. (Optional) If the backup error indicates that your Mac is low on space, enter the following commands one by one:
diskutil list &> ~/Desktop/disk_list.log
diskutil apfs list &> ~/Desktop/disk_apfs_list.log
diskutil info /System/Volumes/Data &> ~/Desktop/disk_info_Data.log
  1. You should then see the following logs on your desktop:
tm_debug.log  
ps.log  
disk_list.log (optional)  
disk_apfs_list.log (optional)  
disk_info_Data.log (optional)
  1. Compress these log files into a single archive (e.g., tm_debug_logs.zip) and send it to TerraMaster support email: support@terra-master.com.