Time Machine backup slow, stuck on "Preparing backup", or never finishing? What it's doing and how to fix it
The bar hasn't moved in an hour, or Time Machine has said "Preparing backup" since lunch. Often it's working through a big job very politely. Sometimes it really is stuck.
What's normal and what isn't
| What you see | Why | What to do |
|---|---|---|
| First backup estimates many hours | It's copying every file on the Mac, at low priority. | Leave the Mac on power and awake overnight. Later backups only copy changes and take minutes. |
| "Preparing backup" for a long time | After a crash, a big macOS update or a long gap, it scans the whole disk to find what changed. | Check tmutil status. If it's moving, wait. |
| Every backup is huge | Something changes big files constantly: virtual machines, video projects, mail or photo libraries, caches. | Exclude what you don't need backed up (below). |
| Very slow over Wi-Fi to a NAS or another Mac | Wi-Fi and network drives are much slower than a cable, and the first backup is big. | Do the first backup over Ethernet or to a directly attached drive, or let it run overnight. |
| "Cleaning up" or deleting old backups | The backup disk is full, so Time Machine removes the oldest backups first. | Normal. If it happens every time, the disk is too small. |
| No progress at all for over an hour, or "Backup failed" | A disk error, a file it can't read, or the drive dropping out. | Follow the unstick steps below. |
See real progress
Open Terminal and run:
tmutil statusLook at BackupPhase, bytes and Percent. Run it again a few minutes later: if the numbers changed, it's working. To read Time Machine's own notes from the last hour:
log show --predicate 'subsystem == "com.apple.TimeMachine"' --info --last 1hRepeated errors about one file or folder tell you what to exclude or fix.
Make backups smaller and faster
- In System Settings, General, Time Machine, click Options and add folders to exclude: Downloads, virtual machine files, game libraries you can download again, big video renders you keep elsewhere.
- Keep the Mac on power and stop it sleeping during the first backup.
- Plug the backup drive straight into the Mac rather than through a hub, and use a cable that supports USB 3. See USB drive copying slowly for how to check.
- Pick a drive about twice the size of the data on your Mac, so it isn't constantly deleting old backups to make room.
- Advanced, temporary:
sudo sysctl debug.lowpri_throttle_enabled=0stops macOS slowing down low-priority disk work, which can speed up a big first backup a lot. It resets when you restart, and the Mac may feel slower while it's on.
Unstick a backup that never finishes
- Click the Time Machine icon in the menu bar and choose Skip This Backup (or Stop Backing Up).
- Restart the Mac.
- Open Disk Utility, select the backup disk and run First Aid.
- Check the disk has free space and is formatted APFS, which Time Machine uses for new backup disks since macOS Big Sur.
- Choose Back Up Now and watch
tmutil status. - If it still fails, try another cable and port. If the disk keeps reporting errors, it may be failing: start fresh on a new drive and keep the old one until the new backup is complete.
Antivirus and cleaning apps that scan the backup disk can also slow or block backups. Excluding the backup disk from their scans usually helps.
Related
Check the backup drive's health · Backup drive clicking or beeping · Mac won't eject an external drive · USB drive copying slowly · Back up phone photos to an external drive · How much cloud storage do I need? · What is a journaling file system? (Wirebook)
Questions
How long does the first Time Machine backup take?
It depends on how much data you have and the connection. Hundreds of GB to a USB drive can take several hours, and much longer over Wi-Fi. Later backups only copy changes and usually take minutes.
Why is Time Machine stuck on Preparing backup?
It's scanning the whole disk to work out what changed, which happens after a crash, a big update or a long gap between backups. Run tmutil status in Terminal to check it's still moving.
How do I speed up a Time Machine backup?
Exclude folders you don't need, keep the Mac on power and awake, plug the drive in directly over USB 3, and do the first backup over a cable rather than Wi-Fi.
Can I use my Mac while Time Machine backs up?
Yes. It runs at low priority so it doesn't get in your way, which is one reason it can seem slow.
What do I do if Time Machine keeps failing?
Skip the backup, restart, run First Aid on the backup disk, check it has space, then back up again. Repeated disk errors mean the drive may be failing.
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