Since January/February 2020, most restarts of my Mac (High Sierra) cause Time Machine to see the complete content of one of my two (internal) drives as new and copy it fully to my Time Machine destination (last Time Capsule model), wiping out my TM history as there isn't enough space to hold all my data twice. The affected drive is a non-boot drive (HDD) and formatted in APFS. A second TM backup to a locally connected drive is not affected by this (this drive is unplugged most of the time).

Analysis of my logs shows the point of failure is the inability to mount the reference snapshot for this drive. Everything works as usual up to and including (generally each entry occurs twice, one for each drive):

  • Mounted stable snapshot: com.Apple.TimeMachine.[date] at path:/Volumes/[...]/[current date]/[drive name] source [drive name]

During a 'normal' backup the following entry is:

  • Mounted reference snapshot: com.Apple.TimeMachine.[date] at path /Volumes/[...]/[previous date]/[drive name] source: [drive name]

When things didn't work as expected (after the restart), only the first internal drive (the boot drive) got this message, whereas the 2nd drive got:

  • Failed to mount APFS snapshot with name: com.Apple.TimeMachine.[previous date] on volume '/Volumes/[drive name]' at mountpoint: '/Volumes/[...]/[drive name]' error: Error Domain=NSPOSIXErrorDomain Code=2 "No such file or directory"
  • Failed to mount reference snapshot: com.Apple.TimeMachine.[previous date]

With APFS-formatted drives, TM uses snapshots by default but if something goes wrong there, it refers back to the eventdb database. That seems be happening here:

  • Forcing deep traversal on source: [2nd drive]
  • Deep event scan at [...] reason: must scan subdirs|require scan|
  • Running deep scan - looking for changes after [previous date]
  • Finished scan
  • Not using file event preflight for /Volumes/[2nd drive name]

Which then results in (corresponding to total used space on that drive):

  • Will copy (1.x TB) from [2nd drive]
  • Do you have the snapshot on your mac? tmutil listlocalsnapshots /path/to/drive? – Joy Jin Dec 4 '20 at 1:14
  • Do you have the mountpoint (/Volumes/[...]/[drive name])? – Joy Jin Dec 4 '20 at 1:52
  • I'll have to wait for the problem to re-occur to check if the requested reference snapshot is completely missing or if it is there but just doesn't want to mount. Right now, there are a couple of snapshots for this drive from the last couple of days. ...................................................... Not sure what you mean with 'having the mountpoint'. Do you mean whether the drive is listed under /Volumes/[drive name]? If that is your question, the drive did show up on the Desktop but I haven't checked whether it was mounted under a different name in \Volumes. – noirdesir Dec 4 '20 at 9:53
  • What I mean is that mounting a snapshot requires a directory that it is mounted on. If that directory does not exist for whatever reason mounting will fail. – Joy Jin Dec 5 '20 at 9:40
  • So if tmutil listlocalsnaphots /Volumes/[drive name] shows the requested reference snapshot, does this mean it has the mountpoint? Since the tmutil man page says **listlocalsnapshots** mount_point List local Time Machine snapshots of the specified volume – noirdesir Dec 5 '20 at 10:17

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