Time Machine couldn't complete the backup macOS Origin: Platform & cloud OSI layer 7 · Application Last updated 5 Nov 2026

macOS: Time Machine couldn't complete the backup. / The backup disk image could not be mounted / Backup failed with error 18/19/21/22

A Time Machine backup stopped because the destination is unavailable, full, or the backup set is damaged.

Exact message you may see Time Machine couldn't complete the backupTime Machine couldn't complete the backup. The backup disk image could not be mountedTime Machine couldn't back up. The backup disk image is already in useTime Machine failed to complete the backupBackup failed with error 18
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Seen on: macOS

Meaning

Check the destination is connected/mounted and has free space, run First Aid, and look at the log: log show --predicate 'subsystem == "com.apple.TimeMachine"' --last 1h. Network (SMB) targets with "could not be mounted" often need re-selecting the disk or deleting the sparsebundle and starting again. Exclusions and corrupted snapshots cause repeated failures.

Common causes

  • Destination full or disconnected
  • NAS/SMB sparsebundle damaged
  • Disk errors on the backup drive
  • Permission/FDA issues
  • Interrupted backups corrupting snapshots

⚡ Quick fix

  1. Run First Aid on the backup disk
  2. tmutil listbackups / delete the broken snapshot
  3. Remove and re-add the destination in Time Machine settings

Detailed fix by platform

macOS

  1. tmutil destinationinfo && tmutil status && tmutil listbackups

How to diagnose

  1. Destination — Mounted, space?
  2. Disk — First Aid result?
  3. Log — Reason?

🔧 Still not fixed?

Many errors look alike. If the steps above didn’t solve it, one of these is probably what you’re facing:

Where this error sits

Origin: Platform & cloud. Errors that are often confused with or caused by this one (click to open):

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