Time Machine stuck on "Preparing backup…" for hours

The Mac went to sleep mid-backup with the drive attached to a hub that dropped out. From then on, every backup attempt sat on “Preparing backup…” — for six hours, overnight, forever. No progress bar, no error. Activity Monitor showed backupd alive and the disk barely ticking over.

Root cause

When a backup is interrupted uncleanly, Time Machine leaves a partial snapshot on the destination as a .inProgress bundle. On the next run it tries to reconcile that half-written snapshot against the whole backup history before doing anything else. On a big, slow USB disk that reconciliation can take so long it’s indistinguishable from a hang — and if the bundle itself is damaged, it genuinely never finishes.

Some of the time you can just wait it out; a “preparing” phase of 30–60 minutes after an interruption is normal. Past a few hours with no disk activity, it isn’t going to recover on its own.

The fix

Delete the leftover partial snapshot and let Time Machine start a clean pass:

1. Turn off automatic backups (or cancel the stuck one) in System Settings → General → Time Machine.
2. Open the backup drive in Finder: Backups.backupdb/<Mac name>/.
3. Find the folder named like 2026-05-12-183014.inProgress and trash it. Don’t touch the dated snapshot folders next to it — those are your history.
4. Empty the Trash and wait for it to finish (deleting a partial snapshot can itself take a while).
5. Re-enable backups and run Back Up Now.

The next backup still spent ~40 minutes in “Preparing backup…” doing a deep traversal — that’s expected after any interruption, because Time Machine re-walks the whole file system instead of trusting its change log. It then backed up normally, and every backup since has been the usual few minutes.

Notes

• This applies to HFS+ Time Machine destinations (the classic Backups.backupdb layout, typical for USB drives formatted before Ventura). APFS destinations store backups as snapshots and don’t expose an .inProgress folder — there, an interrupted backup usually just needs the long deep-scan pass, so give it real time before assuming a hang.
• If backups keep getting interrupted, check the physical path first: direct-attach the drive instead of going through a hub, and disable “Put hard disks to sleep” in Energy settings.