BackupA Time Machine backup that's genuinely stuck and one that's just slow look identical: a progress bar that isn't obviously moving and an ETA you don't believe. Here's how to tell them apart, what actually causes stalls, and the part Apple never fixed — that Time Machine will happily stay broken for days without telling you.
The 30-second stall test
Open Terminal and run:
tmutil statusLook at bytes under Progress. Wait 30 seconds and run it again.
bytesmoved → it's slow, not stuck. Let it run.bytesidentical → it's stalled. Something below applies.
What actually causes stalls
1. The backup disk dropped off. External drives disconnect under sustained write load (power, cable, enclosure heat) — and when that happens mid-backup, Time Machine doesn't error. It sits in "Copying" forever, waiting for a disk that's gone. Check whether your backup drive is still mounted.
2. Cheap-SSD cache collapse. Consumer external SSDs write fast until their internal cache fills (often 10–30 GB into a big backup), then drop to a crawl while it flushes. Looks exactly like a stall; the two-sample test shows bytes still creeping. Patience — and expect it on first backups, not incrementals.
3. A sleeping network destination. NAS or Time Capsule backups stall when the device naps or the Wi-Fi path degrades.
4. Your startup disk is nearly full. Time Machine needs roughly 10 GB free on your Mac to stage each backup. Below that it fails in genuinely cryptic ways — sometimes blaming random files, sometimes hanging. If your backups broke mysteriously, check your own disk's free space first. (This one is wildly underdiagnosed.)
5. First-backup "Preparing" that lasts hours. After OS upgrades or long gaps, deep traversal is normal. The stall test still applies — file counts move even when bytes don't.
The bigger problem: silence
The worst part isn't the stall — it's that nothing tells you. Time Machine's menu bar icon looks the same whether your last backup was an hour ago or nine days ago. It can report "backup completed" without a snapshot actually landing on the disk. Most people discover weeks of missing backups at the exact moment they need one.
Getting told, automatically
This silence is the reason BackupTempo exists (free). Beyond scheduling Time Machine on your terms, it:
- Verifies every backup against the destination's own records — a "finished" backup that didn't produce a snapshot is reported as a failure, not celebrated
- Watches for stalls — a frozen byte count during copying triggers a plain-language alert ("the backup disk may have disconnected")
- Pre-flights every backup — checks your startup disk's staging space and the destination's presence before trying, and tells you the real reason when it won't work
- Wears its status in the menu bar — orange after 24 hours without a verified backup, red after 48 or repeated failures, before you've lost a meaningful amount of history
You shouldn't need to run tmutil status in Terminal to know your backups work. Download BackupTempo and let the menu bar tell you.