If your Time Machine backups started failing right after updating to macOS Taho, you’re not alone. The pattern is very specific: the initial backup never finishes, there’s no clear error, reformatting the external drive doesn’t help, and the same setup worked perfectly on the previous macOS version.
That combination tells us something important right away. This is almost certainly not a file system problem, not a permissions issue, and not user misconfiguration. This is a low-level interaction problem between Time Machine, APFS snapshots, power management, and certain external SSD enclosures.
Let’s break down what’s actually happening, how to confirm it, and what you can realistically do about it.
What changed in macOS Tahoe (and why Time Machine is affected)
Time Machine behaves very differently during the first backup compared to incremental ones. The first run triggers a full APFS snapshot crawl of your entire system volume. This is the most I/O-heavy and long-running operation Time Machine ever performs.
In macOS Tahoe, Apple tightened two things aggressively on Apple silicon Macs:
- Background task throttling
- External disk sleep and power state transitions
These changes are good for battery life, but they exposed a weak spot.
Some external SSD enclosures, including several SanDisk models, have firmware that does not handle repeated sleep, wake, or UASP resets cleanly during long sustained reads. When this happens mid-snapshot scan, Time Machine doesn’t always throw an error. Instead, it stalls silently. No progress, no failure message, just endless “preparing” or “backing up”.
That’s why reformatting the drive doesn’t change anything. You’re hitting a transport-level failure, not a file system issue.
Why generic fixes don’t work here
You’ll see plenty of advice like:
- Reformat as APFS
- Remove and re-add the backup disk
- Try a different partition layout
- Reset Time Machine preferences
Those steps are fine in general, but in this case they miss the root cause. Formatting only affects the structure on the disk. It does nothing to fix how the enclosure firmware behaves when macOS aggressively manages power during long snapshot operations.
If Time Machine never finishes the initial backup after hours, that’s your signal. This isn’t normal behavior and it’s not something you can brute-force with retries.
How to confirm this is the real problem
Before changing anything, it’s worth verifying what Time Machine is actually doing.
1. Watch Time Machine at the command line
Connect the external drive directly to your Mac. No hubs, no docks.
Then run:
tmutil startbackup –block –verbose
This forces Time Machine to run in the foreground and shows exactly where it pauses.
If you see it progressing normally for a long time and then sitting idle with no disk activity, that’s a classic stall. If it were a permissions or format issue, you’d get an immediate error instead.
2. Check Time Machine logs
Right after a failed or stalled attempt, run:
log show –predicate ‘subsystem == “com.apple.TimeMachine”‘ –last 1h
If you see repeated messages like:
- snapshot invalidated
- device not responding
- I/O timeout during snapshot
you’ve confirmed the diagnosis. This is a snapshot + device interaction failure, not a configuration mistake.
One workaround that sometimes helps (but only once)
There is one thing worth trying because it addresses the power-management angle directly.
Disable external disk sleep temporarily:
sudo pmset -a disksleep 0
Then reboot your Mac and attempt the initial Time Machine backup again with the drive directly connected.
Why this can help: it prevents macOS from putting the external disk into a low-power state mid-scan. On some enclosures, that’s enough to let the initial snapshot crawl complete.
Important caveat:
If this fails once, repeating it won’t magically fix things. At that point, you’re dealing with a firmware incompatibility.
Why ChronoSync works when Time Machine doesn’t
Your experience with ChronoSync is actually a useful data point.
ChronoSync performs file-level copy and sync operations. It does not rely on APFS system snapshots in the same way Time Machine does, and it doesn’t hold the disk open in a single massive transaction for hours.
That’s why it works reliably even when Time Machine doesn’t. It avoids the exact stress pattern that triggers the enclosure firmware bug.
Using ChronoSync as a short- to medium-term backup solution is perfectly reasonable here. You’re not doing anything unsafe or incorrect.
Long-term solutions that actually make sense
Once you’ve confirmed this behavior, the realistic options narrow quickly.
Option 1: Use a different enclosure or drive
This is the most reliable fix today.
Different enclosure firmware behaves very differently under sustained APFS snapshot load. Even moving the same SSD into another enclosure can solve the issue instantly.
If you want Time Machine back now, this is the fastest path.
Option 2: Wait for a macOS point update
Apple has adjusted Time Machine snapshot retry logic in past releases when issues like this surfaced. It’s very possible that a future Tahoe update will relax snapshot retries or handle stalled devices more gracefully.
There’s nothing you can do to force this timeline.
Option 3: Keep ChronoSync and move on
This is not a failure or compromise. Plenty of professionals rely on scheduled file-level backups alongside periodic manual system images.
If your data is protected and verified, you’re doing the job correctly.
What not to waste time on
Based on real-world behavior, these are dead ends in this scenario:
- Reformatting the drive repeatedly
- Switching between APFS and other formats
- Partitioning tricks unless you change hardware
- Third-party “disk utilities” bundled with drives
- Reinstalling macOS
None of these touch the underlying problem.
Final takeaway
This Time Machine failure on macOS Tahoe is not user error. It’s not a formatting mistake. It’s not something Apple Support boilerplate will fix.
It’s a low-level interaction between Tahoe’s stricter power management, APFS snapshot behavior on Apple silicon, and certain external SSD enclosure firmware that can’t handle sustained snapshot scans without choking.
Once you understand that, the situation becomes much clearer and a lot less frustrating. You’re not stuck. You just need to decide whether to change hardware, wait for a macOS update, or continue with a backup tool that already works for you.
And no, reformatting again won’t help.
