[Solved] Time Machine Backup Hanging on ‘Preparing Backup’ on macOS Sonoma 14.7: Diagnostic steps and permanent fix

[Jeremy Lehmann]

Diagnostic Hardware Specs: MacBook Pro 14″ M3 Max | OS Version: macOS Sonoma 14.7 | Archive Source: Apple Support Community #256295425

I am running into a persistent issue on my MacBook Pro 14″ M3 Max running macOS Sonoma 14.7.
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?

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

Smart Home & Matter/Thread Integrator | "Verified Apple Discussions Archive"

[Scott Mason]

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 Pro (48GB) | Studio Display | HHKB Professional Hybrid | Miami, FL

Apple Certified Support Professional (ACSP) | Computer Science & Machine Learning Student

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