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Facing an issue with "Time Machine couldn’t verify your backups" on macOS Sonoma.

Similar questions have been asked around but always end with absolute random solutions that work for someone:

I'm interested to find a way how to systematically debug Time Machine errors like this, such as accessing Time Machine logs, etc. Are there any best practices for this?

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  • You’re backing up to a network share? I would try to see if you can do your first backup locally to an attached drive. Then you know if the system has issues (incompatible third party apps or bugs, corrupt file system) or it’s the quality of your network and NAS. Also, go directly to Howard Oakley’s Time Machine mechanic tool (referenced in Gilby’s good answer).
    – bmike ♦
    Commented Jul 9 at 11:07
  • @bmike Didn't mention that I used to backup to this network share for about a year with no issue. It stopped working a few months ago and since then I wasn't able to figure our why..
    – adamsfamily
    Commented Jul 9 at 17:41

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My practices (best or otherwise):

  1. Ignore all technical descriptions, discussions and solutions more than about 2 years old. The internals of Time Machine (TM) have changed in two major ways:

    • Old TM used HFS+ formatted disks and avoids duplication of files used hard links. Anything related to this (like all three of your 12+ year old links) is useless today.

    • Current TM uses APFS formatted disks which uses "snapshots" for reliability and avoiding duplication of files in each backup. The internals of this have changed (e.g. checksumming of files) further limiting applicability of say 4 year information.

  2. TM to network attached drives (whether another Mac or a NAS) are notoriously fragile. In cases like yours, using a Synology NAS, all questions need to make that clear what NAS is being used as well as having already ensured that the NAS explicitly supports TM and is appropriately configured.

  3. Logging (and this is the crux of your question): Time Machine writes to the unified log - it does not have a separate log. The unified log is voluminous (to say the least) and it is difficult to make any sense of it. Nevertheless:

    • Howard Oakley has written a range of utilities to assist. Time Machine Mechanic, Mints and Consolation are introduced on this page T2M2, etc.

    • Start with The Time Machine Mechanic. Here is when there has been one successful backup:

      TMMechanic screenshot showing "✔ No error messages found."

  4. Read and, hopefully, understand the many posts regarding Time Machine towards the end of the above linked page. Some depth of understanding how TM works is essential to diagnosing issues. Time Machine in Sonoma detection and diagnosis is directly relevant.

  5. When using TM to another Mac I have always had trouble and decided that pursuing the issues is wasted effort. I have turned to directly attached storage even if that is only intermittently connected - e.g. to a MacBook. YMMV - others do successfully backup to Synology.

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  • Thanks for the detailed comment! I forgot to mention that I did back up to this Synology share with no issues for about a year. Things went south a few months ago, since that time I've removed all my snapshots, removed the entire backup and configured TM again on the same Synology share. Synology hasn't really been touched for more than a year, therefore I tend to look for problems on my Mac. I'll try T2M2 first, thanks for the tips!
    – adamsfamily
    Commented Jul 9 at 17:46
  • Marked the answer as accepted, as it addresses my question: how to debug Time Machine errors. Now I got some insights, my problem is not solved yet but I wouldn't clutter this question with issues specific to myself. Thanks!
    – adamsfamily
    Commented Jul 10 at 19:04

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