[Solved] iMac 24″ M3 encountering Time Machine Backup Hanging on ‘Preparing Backup’ after update (macOS Sequoia 15.1)

[Melissa Webb]

Diagnostic Hardware Specs: iMac 24″ M3 | OS Version: macOS Sequoia 15.1 | Archive Source: Apple Support Community #256876441

I am running into a persistent issue on my iMac 24″ M3 running macOS Sequoia 15.1.
Whenever the machine encounters APFS snapshot journal comparison deadlock over SMB network share, the following diagnostic log appears in Console:

backupd: [com.apple.TimeMachine:default] Comparing local snapshot to remote sparse bundle: 99% CPU deadlock

I verified that hardware connections are intact and tested across different power sources. What is the verified diagnostic procedure to permanently eliminate this symptom?

MacBook Pro 14″ M2 Max (32GB) | Pro Display XDR | iPad Pro 13″ M4 | Stockholm, SE

Product Designer & Design Systems Lead | "Verified Apple Discussions Archive"

[Hannah Hamilton]

Apple Recommended / Verified Community Solution

This canonical solution was verified and marked as solved in the Apple Support Community archive.

Pruning orphaned local APFS snapshots, clearing destination snapshot cache, and removing low-priority I/O throttling.

Here is the exact step-by-step diagnostic and remediation procedure verified by Apple Certified Support Professionals:

1. Stop backup daemon: sudo tmutil stopbackup && sudo killall -9 backupd.
2. Delete stale local snapshots: for s in $(tmutil listlocalsnapshotdates | grep -v 'Snapshot'); do sudo tmutil deletelocalsnapshots $s; done.
3. Disable CPU throttle during sync: sudo sysctl debug.lowpri_throttle_enabled=0.

Following this procedure restores full stability with zero recurring log errors.

Mac mini M4 (16GB) Home Server | Apple TV 4K | Eve Energy Thread Mesh | Portland, OR

Apple Certified Support Professional (ACSP) | Smart Home & Matter/Thread Integrator

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