Time Machine backup failed? Check the disk before erasing it

For a directly attached disk, plug it into the Mac without a hub and check the cable and power. For a network disk, check that the Mac is on the same network and can reach the backup destination. If you already have a successful network backup, hold Option while opening the Time Machine menu and choose Verify Backups to check it.

Check the space on your Mac as well as the backup destination. A nearly full startup disk can leave too little room for the temporary data needed to finish a backup. If Time Machine says the destination is too small, choose a larger disk or exclude only files you can safely leave out. Apple recommends a backup disk with at least twice the Mac’s storage capacity.

Keep the Mac connected to power and the internet while cloud files finish syncing. If Time Machine reports that files were unavailable, try a manual backup with the Mac unlocked. Then use the Time Machine menu to check the latest completed backup date. A backup that is still in progress may just need more time, especially on its first run.

If the external disk still reports errors, follow Apple’s Disk Utility First Aid instructions and verify the disk you select. If it cannot be repaired, prefer a different backup disk when you need to keep the only copy of older files. Erasing the old disk removes its backup history. Check that a new backup completes before you stop relying on the old disk.