>

Why does my Time Machine backup keep failing?

Diagnose and fix recurring Time Machine backup failures on macOS from the Terminal: check status, verify the destination, and confirm a completed backup.

macOSbackuplogtroubleshootingtmutilloggingtime-machineMedium risk
Run tmutil status and tmutil latestbackup to see whether Time Machine is erroring mid-run or hasn't backed up at all.

What this covers

When Time Machine fails repeatedly, the cause is almost always one of three things: a dead or missing destination, a stuck run, or one problematic folder that never syncs. This guide walks through all three from the Terminal — no reinstalling, no guesswork.

Symptom

?Time Machine reports errors or retries endlessly, backups haven't completed in days, or the backup disk keeps getting unmounted mid-run.

Check what Time Machine is doing right now

Before changing anything, find out whether a backup is stuck, erroring, or simply hasn't run. Checking the current status and the timestamp of the newest backup tells you if Time Machine is failing mid-run or hasn't backed up in days. If the status shows an error or the latest backup is far older than your backup interval, you have a real failure to dig into.

macOSCheck Time Machine's Latest Backup with tmutilLow riskStep 1

Two tmutil queries answer 'is Time Machine working?' — status shows the running state, latestbackup the newest backup.

Command
$ tmutil status && tmutil latestbackup
Full explanation & undo →

Confirm the backup destination is still valid

Many 'keeps failing' cases come from a destination that no longer resolves: a renamed disk, an unplugged drive, or a network share that changed. Checking the destination info shows which volume Time Machine is pointed at and whether it's available. If the destination is missing or shows as unavailable, that's your root cause.

macOSCheck Where Time Machine Backs Up with tmutil destinationinfoLow riskStep 2

Find out which disk or network share Time Machine is using as its backup destination, straight from the terminal.

Command
$ tmutil destinationinfo
Full explanation & undo →

Search the unified log for the actual error

If the status doesn't make the cause obvious, the system log usually does. Filtering the unified log for the last hour surfaces kernel-level messages from around the time the backup failed — disk I/O errors, disconnections, or space problems. What you find here decides whether the fix is the destination, the disk, or the data.

macOSSearch the Unified Log with log showLow riskStep 3Optional

Filter macOS's single unified log by process, message type, or subsystem straight from the Terminal.

Command
$ log show --last 1h --predicate 'process == "kernel" AND messageType == 16' --style compact
Full explanation & undo →

Re-point Time Machine at a healthy destination

If the destination was missing or pointing at a dead volume, set it again to the correct backup volume (or add a second rotation disk). This recreates the link Time Machine needs. It worked when the destination shows up correctly and Time Machine stops complaining about a missing disk.

macOSSet the Time Machine Backup Destination with tmutilMedium riskStep 4

Point Time Machine at a backup volume or add a second rotation disk directly from the command line.

Command
$ sudo tmutil setdestination -a /Volumes/BackupDisk
▾ Undo
Undo
$ sudo tmutil removedestination <destination-uuid>
Full explanation & undo →

Exclude folders that break or bloat the backup

Backups often fail because one huge or constantly-changing folder — VM images, caches, build directories — never finishes syncing or fills the disk. Excluding such folders lets every other backup complete. Add exclusions for anything volatile that you don't actually need to restore.

macOSExclude a Folder from Time Machine with tmutil addexclusionMedium riskStep 5Optional

Keep volatile folders like VM images and caches out of every backup.

Command
$ sudo tmutil addexclusion -p "/Users/apple/Downloads/虚拟机镜像"
▾ Undo
Undo
$ sudo tmutil removeexclusion "/Users/apple/Downloads/虚拟机镜像"
Full explanation & undo →

Kick off a backup now and watch it run

Don't wait for the next scheduled run — start a backup immediately to confirm the fix. Watch it progress without erroring out. If it stalls again, go back to the log step; if it completes, the failure is resolved.

macOSStart a Time Machine Backup Now with tmutilLow riskStep 6

tmutil startbackup kicks off a backup immediately — handy before system updates or big changes.

Command
$ tmutil startbackup
▾ Undo
Undo
$ tmutil stopbackup
Full explanation & undo →

Confirm the new backup actually landed

Finally, list all backups and check that a new dated entry appears at the bottom. That dated folder is proof the backup completed and is restorable. From here on, Time Machine should keep producing new entries on schedule.

macOSList All Time Machine Backups with tmutil listbackupsLow riskStep 7

Print the mounted path of every dated backup on your Time Machine disk, newest last.

Command
$ tmutil listbackups
Full explanation & undo →
!Re-adding a destination or deleting snapshots affects your only safety net — make sure at least one good backup exists before removing anything.

How to confirm it worked

Start a manual backup, let it finish without errors, then confirm a new dated backup appears in the backup list.

Undo everything

Every command above can be reverted. Run these in order to undo all of it.

Undo everything
$ tmutil stopbackup
sudo tmutil removeexclusion "/Users/apple/Downloads/虚拟机镜像"
sudo tmutil removedestination <destination-uuid>

Review before running — some steps may not be independent.

Sources

Last verified: 2026-09-21