Time Machine backups hanging at roughly 39% on both a Synology NAS (over Ethernet) and a direct USB drive point to a Mac-side system, spotlight/indexing, or file-corruption bottleneck rather than a pure network or NAS hardware failure.
Direct Troubleshooting Steps:
Ensure Your Mac Remains Unlocked
Certain files require your Mac to be unlocked for access, and that can stall a backup when the Mac is locked. If that's the cause of the hang, then Time Machine should show an error message reflecting that (I have seen it on one of my kids' Macs). In that case, you can set a Hot Corner to prevent the screensaver from starting and use that during a backup.
Use hot corners on Mac - Apple Support
Check and Exclude Problematic Files (Mac-Side)
Time Machine often hangs when it hits a corrupt file, a locked system database, or a rapidly changing virtual disk/cache file (such as Parallels/VMware images, large local Docker volumes, or encrypted sparse bundles).
- Open System Settings > General > Time Machine > Options.
- Temporarily add large virtual machine files, heavy development folders, or cloud sync directories (Dropbox, Google Drive) to the exclusion list.
- Run a backup to see if it moves past 39%.
Verify Spotlight Index Corruption
Time Machine relies heavily on the macOS Spotlight index to scan changes. If the index is corrupt, the backup engine freezes during the "preparing" or mid-transfer phase. Try rebuilding your index.
Rebuild the Spotlight index on your Mac - Apple Support
Check for Antivirus or Security Software
Third-party security tools intercept Time Machine's sparse bundle writes and cause infinite stalls.
- Temporarily disable or completely uninstall security/antivirus apps like Bitdefender, Norton, Sophos, or Malwarebytes.
- Test booting your MacBook Pro in Safe Mode to isolate whether a background kernel extension or login item is locking the backup stream.
Start up your Mac in safe mode - Apple Support
Check Other Users
One user here recently reported that TM backup hangs were caused another user account on their Mac that was an unused family member account transferred by Migration Assistant. Deleting that legacy user account fixed the issue.
Delete a user or group on Mac – Apple Support
Inspect Terminal Debug Logs
Instead of guessing, pull the live Time Machine error log while the backup is hung:
log show --predicate 'subsystem == "com.apple.TimeMachine"' --info --last 1h
- Look for specific I/O errors, permission denials, or timeout messages around the 39% mark
Personally, I have a Synology DS223j (primary) and a pair of large USB-C SSDs (secondary to swap offsite) that I use for TM backups of four Macs in the house. Two of the Macs are running Golden Gate, and TM backups are working file for all the Macs.
User level: Level 1 (4 points)
