Symptoms
- Time Machine shows an error message about the backup failing
- Backup progress stalls at the same point every time
- Backup drive disconnects unexpectedly during the process
- Last successful backup was a long time ago
Likely causes
- CommonThe backup drive is full or nearly fullTime Machine needs room to manage both new backups and older ones it may need to delete.
- CommonThe backup drive disconnected or went to sleep during the backupThis is especially common with network drives or drives on a hub.
- PossibleA corrupted backup requiring a fresh startLong-running backup drives can accumulate errors over years of use.
- PossibleA weak or unstable network connection for network-based backupsWi-Fi backups are more prone to interruption than a directly connected drive.
- Less commonThe backup drive's format or connection type having an issueSome external drives develop connection or formatting problems over time.
What to do
- Check that the backup drive is properly connected and powered on, and that its cable is secure.
- Check available space on the backup drive in Finder; Time Machine needs it to be larger than your used storage and to have room to work.
- Open Time Machine preferences to see the specific error message, which often points directly to the cause.
- If backing up over Wi-Fi, try a direct connection temporarily to see if that resolves it.
- Restart the Mac and try running the backup manually from the Time Machine menu bar icon.
- If a specific backup keeps failing, consider starting a new backup on the same drive (Time Machine will use available space) rather than continuing to troubleshoot the old one.
- Run Disk Utility's First Aid on the backup drive to check for and repair errors.
- If the drive shows persistent errors, back up your important files elsewhere and consider replacing the backup drive.
Tools and parts
- None required
External hard drive or SSD for backupsSearch: “external drive for Time Machine backup”
When to call a pro
If Disk Utility can't repair the backup drive or backups keep failing on a healthy drive, a technician can help diagnose whether it's the drive, cable, or Mac causing the issue.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.