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

**Code:** `Time Machine couldn't complete the backup` · **Category:** macOS · **Seen on:** macOS

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

## 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?

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Source: [WhatIsMyError](https://whatismyerror.com/errors/macos/time-machine-backup-failed)