Time Machine erased 18 months of backups without warning

TidBITS Talk
42

Smart Update.

43

In that case, and I’m not at my Mac to verify this…

I think for full volume exclusions, Time Machine stores the volume’s unique ID (UUID) in /Library/Preferences/com.apple.TimeMachine.plist, under the ExcludedVolumeUUIDs key.

So possibilities include:

  • Time Machine lost the UUID from this preference file
  • The drive’s UUID changed
  • You didn’t exclude the entire volume
  • Bug somewhere

If the entire volume isn’t excluded then it is a path-based exclusion. And I think this could be trouble for an external drive, because the path can change. For example, if you mount a drive named Mike, the path is /Volumes/Mike/. But if you already had a drive named Mike mounted then the new drive’s mount path is /Volumes/Mike 2/. That’s not the same path, so it wouldn’t be excluded.

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44

Thanks. Here is the list of excluded UUIDs – some of these go back years and are not actively used.

	<key>ExcludedVolumeUUIDs</key>
	<array>
		<string>799A5B1F-260F-317C-8903-9FEFA4B4C021</string>
		<string>631D2D8A-C5A8-4C79-BC5E-998758383839</string>
		<string>718CF01F-66C8-3A6D-BCC7-DF4A7B56D5EF</string>
		<string>A0AB127C-9E43-40EA-B31C-402162A1E2FB</string>
		<string>70E3FA08-64BA-32E2-8C08-113ED11F3757</string>
		<string>DF80DEE8-E556-36C4-B6E2-F5A409BC6241</string>
		<string>DAE7BE5E-C487-3338-B0DA-A92CF01B192D</string>
		<string>2EE9F979-7B52-3734-9CCC-225F6300C395</string>
		<string>75C67034-7F0E-3A29-BD57-8F234BA286A4</string>
		<string>638E3EE6-1769-4C9B-96F0-91EB3C8F9B5F</string>
	</array>
	<key>ExcludedVolumes</key>

That last one matches what diskutil shows for my backup drive (B8-Backup):

diskutil list
/dev/disk9 (synthesized):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      APFS Container Scheme -                      +4.0 TB     disk9
                                 Physical Store disk6s2
   1:                APFS Volume B8-Backup               11.3 GB    disk9s1
   2:                APFS Volume Preboot                 25.0 GB    disk9s2
   3:                APFS Volume Recovery                1.3 GB     disk9s3
   4:                APFS Volume VM                      20.5 KB    disk9s4
   5:                APFS Volume B8-Backup - Data        2.4 TB     disk9s5

diskutil info /dev/disk9s5
Volume UUID:               638E3EE6-1769-4C9B-96F0-91EB3C8F9B5F

Did I exclude the correct volume (B8-Backup - Data)? I simply dragged the Desktop item onto the TM exclusion list.

Note, however, that I also have /Volumes in the TM exclusion list so this should exclude any mounted drive that is not the current system/boot drive.

Thanks, again, for the suggestions and ideas.

45

If you are using more than one TM drive for backups (IMHO a good strategy) then it is possible there is some corruption, causing a difference between them, that is confusing TM. It won’t be easy to check this, except maybe by a process of elimination. For example, you can remove a drive from TM Settings and add it back later after doing some diagnostics. TM should pick up where it left off.

As discussed above, I would certainly ensure I had a separate backup such as CCC, before experimenting with TM settings.

46

No, only one drive for TM.
I have TM, SuperDuper, and CCC.

I think it is time to erase my TM drive (TM_06) and start over. I’ll try to monitor more closely this time to see if I can catch anything useful in diagnosing this.

47

I just want to make one more plug for CCC here, because you already have it. CCC does backup versioning, just like Time Machine, but with more control. I’m not talking about “SafetyNet” either, CCC creates snapshots. You can control how long they’re kept on your backup disk, you can even tell CCC to never delete snapshots (granted, you’d eventually run out of space, but it’s your choice). My CCC backup is a “clone” insofar as the version I can see in the Finder is exactly what the source looks like, but then I also have snapshots dating back to 2024.

I’d take a moment to see if you have that enabled on your CCC backups, maybe you don’t need Time Machine at all. Here’s a reference to the CCC docs: https://support.bombich.com/hc/en-us/articles/21384373407127-Snapshot-thinning-and-pruning

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