Delete ghost volume from Time Machine snapshot

March 8, 2022

This is the blog post version of my answer to this Stack Exchange question.

Why I needed to remove a volume backup

Recently, the external hard drive where I kept some of my video footage died.

But as I’m a good citizen and I have a backup (and restore) strategy, all the data was also mirrored on my Time Machine drive! Yay!

So I proceeded to restore it to another internal drive where I had enough room.

This could be the end of the story. But Time Machine kept persisting the backups of that old external drive in all further snapshots.

This is a nice feature, for example if your external drive is unplugged for a few days, you don’t want Time Machine to remove it from your backup history. But in my case, that drive was actually dead and now I restored it, I didn’t need this redundant “phantom” backup.

It turns out you can’t easily delete arbitrary directories in a Time Machine backup, and the tmutil delete command doesn’t let you delete granular parts of a backup. It’s either a whole snapshot, machine directory or backup store.

Luckily, by messing around with the tmutil command, and because I already played a bit with it, in the past I figured a way to hack it to remove a specific ghost volume from a backup!

The theory

Time Machine has in its state a directory Video (in my case) in the backup snapshots, which it associates with /Volumes/Video, or more specifically, the original disk UUID behind this mount point. Because that disk is dead, this UUID is never to be seen again. But Time Machine can’t know that. From its perspective, it’s just like this external drive is unplugged, and it’s a good thing that it doesn’t remove it form backups!

So, if we tell Time Machine that the Video backup directory is now associated with another disk (that actually exists), it will effectively put the new backup in it, instead of carrying over the backup of the dead disk.

And even better, if we associate a disk that is excluded from Time Machine backups, it will delete the Video directory altogether from new snapshots!

Removing the ghost backup directory

First, we need to identify a volume that’s already excluded from Time Machine. Go in the Time Machine preferences, and in Options…, see if you have an excluded volume. It could be an internal drive that you explicitly excluded from backups, or an external drive that you never explicitly included in backups.

If you don’t have any excluded disk (it needs to be a disk, not a subdirectory), you can plug a random USB stick or SD card or something similar.

Let’s pretend that your excluded disk is a USB stick, in /Volumes/USB, and I’m trying to get rid of the Video directory inside my future backup snapshots:

sudo tmutil associatedisk /Volumes/USB "/Volumes/{TimeMachineDrive}/Backups.backupdb/{MachineDirectory}/Latest/Video"

Now Time Machine thinks that the Video directory is associated with /Volumes/USB, which is excluded, and so, it will exclude it from future backups!

The history of the dead Video drive will still be present in the older snapshots, but Time Machine will be able to reclaim the space from it in the future because that “ghost” volume is not being referenced anymore in the newer backups.

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