Why has my Time Machine backup failed or got stuck?
Stuck on preparing, failing with a verification error, or running for days without finishing. The five things that actually cause it, in the order worth checking, and when a backup history has to be abandoned rather than repaired.
- Difficulty
- intermediate
- Time
- 40 min
- Read
- 6 min
- Safety
- caution
Short answer
Check the date of the last successful backup first — that tells you how urgent this is. Then work through the four common causes in order: the disk is full and thinning has failed, the disk or its file system is faulty, something enormous is being backed up over and over, or the backup is genuinely just slow because it is the first one. A backup that reports the destination needs to be recreated has failed verification, and that history cannot be repaired.
Time Machine is unusually good at running invisibly and unusually bad at telling you when it has stopped. It reports failures in a notification that is easy to dismiss, and then carries on failing silently, so the common experience is discovering months later that the last successful backup was in the spring. That is the reason the first step here is checking the date rather than reading the error. Once you know how exposed you are, the diagnosis is tractable: Time Machine failures cluster into a small number of causes, and the error messages, unhelpful as they look, do distinguish between them.
Safety
Step by step
- Check when the last successful backup was.Hold Option and click the Time Machine menu, or open its settings, to see the last completed backup. If that is weeks or months ago, stop and make a manual copy of your important folders to another drive right now. Fixing Time Machine is the second priority.
- Check free space on the destination.Time Machine deletes its oldest backups to make room, but it needs enough headroom to work with and it fails rather than deleting aggressively when a single backup would not fit. A destination that is smaller than about twice the data being backed up will eventually produce exactly this.
- Look at what it is trying to back up.A virtual machine image, a large mail archive, a photo library, or an encrypted disk image changes wholesale every time it is touched, so Time Machine copies the entire thing at every backup. One such file can make every backup take hours. Excluding it and backing it up separately is the standard remedy.
- Verify the destination disk.Run First Aid on the backup disk in Disk Utility. A backup drive that has developed file system errors will fail in ways that look like Time Machine's fault. External drives that live plugged in and get knocked, or are unplugged without ejecting, accumulate these.
- Distinguish slow from stuck."Preparing backup" genuinely takes a long time when there are many files or after an interruption, because it is comparing the whole file list. Watch the byte counter rather than the phase — a counter that is moving, however slowly, is not stuck.
- Accept when a backup history has to be abandoned.If Time Machine reports that it must create a new backup because the existing one cannot be verified, the history is not repairable. The right response is to start a fresh backup, ideally on a different disk, and keep the old one read-only until the new one has completed successfully at least once.
- Reconsider the destination if failures keep recurring.Network destinations — a drive attached to a router, or a third-party network drive — are far more failure-prone than a directly attached disk. Apple supports Time Machine over its own network protocol to properly supported destinations; anything improvised is a common source of exactly these errors.
- Make sure Time Machine is not your only backup.It is a good local backup and it fails silently, which is a bad combination on its own. The catalogue's guide on setting up a proper backup covers the three-copy principle, and a second copy somewhere else is what turns a Time Machine failure into an inconvenience rather than a loss.
Common mistakes
- Dismissing the failure notification without reading it — It is the only warning you get, and Time Machine then continues failing silently. The single most valuable habit here is checking the last-backup date once a month.
- Treating Time Machine as your whole backup — It is one local copy on a disk that lives next to the computer. Fire, theft, ransomware and a dropped drive take both at once. It needs a companion somewhere else.
- Reformatting the backup disk at the first error — That throws away the entire history, which may still be perfectly restorable, before you know whether the fault was space or a cable. Verify before destroying.
If it doesn't work
"Time Machine could not complete the backup" with no detail
Cause: Space, permissions or a file that cannot be read — Fix: Check destination free space first. If space is fine, run First Aid on both the source and the destination. Console logs name the failing file if you need to go further.
Says the backup must be recreated
Cause: The existing backup failed verification — Fix: That history cannot be repaired. Start a new backup, preferably on a different disk, and keep the old one untouched until the new one has completed.
Stuck on "Preparing backup" for hours
Cause: A long file comparison, or Spotlight indexing the destination — Fix: Give it several hours after an interruption or a macOS upgrade. If it never progresses, excluding the backup disk from Spotlight and restarting usually releases it.
Every backup is enormous
Cause: One large file changing wholesale — Fix: Look in the Time Machine exclusion list and add virtual machine images, disk images and large database files. Back those up separately with a method that suits them.
Backups only fail over the network
Cause: The network destination — Fix: Test with a directly attached USB drive. If that works reliably, the network destination is the problem — many third-party network drives support Time Machine only loosely.
External drive keeps disconnecting mid-backup
Cause: Power, cable or sleep settings — Fix: A bus-powered drive on a hub is the usual cause. Use a direct port or a mains-powered drive, and check that the Mac is not sleeping the disk partway through.
Questions people ask
How big should my Time Machine disk be?
At least twice the amount of data you are backing up, and more is better because the extra space is what buys you history rather than just a current copy. A disk barely larger than your data will keep working but will hold only a few days of versions.
Can I use the same disk for Time Machine and ordinary files?
You can partition it and use one part for each, and it is a poor idea. A backup disk should be reserved for backups: sharing it means the backup competes for space and your working files sit on the same physical device that is meant to protect them.
Does Time Machine back up to iCloud?
No — Time Machine is local or network-attached only. iCloud Drive syncs files, which is a different thing, and the catalogue's guide on whether cloud sync is a backup explains why the distinction matters. If you want an off-site copy of a Mac, that is a separate backup service.
Is a failed backup a sign my Mac's drive is failing?
Occasionally. A source-side read error will fail a backup, and repeated failures naming different files is a genuine warning sign. Running First Aid on the internal drive, and checking its health, is worth doing if the destination checks out clean.