Friday, July 11, 2025

Using “tmutil associatedisk” With APFS Destinations

I recently got a new SSD on Prime Day to replace one of my main hard drives. As this drive was included in Time Machine, I wanted the SSD to “adopt” the hard drive’s backup history. This way I could avoid recopying lots of data that was already backed up, which would also require pruning older snapshots.

When you get a new Mac and want to adopt the old Time Machine backup, you want tmutil inheritbackup. When you keep the same Mac but get a new source drive, you want tmutil associatedisk.

The command is documented as:

tmutil associatedisk [-a] mount_point snapshot_volume

The -a tells it to find all the snapshots for that volume on the destination, not just the specific one that you pointed it to.

mount_point is just the source volume’s path (in /Volumes, not the device path).

snapshot_volume is the destination within your Time Machine backup. The example shows this as being within the Backups.backupdb folder, but there’s no such folder when using an APFS destination. My first thought was to drag the latest snapshot from Finder into Terminal:

sudo tmutil associatedisk -a /Volumes/Aux /Volumes/.timemachine/C2E8322E-A7EA-44F8-904F-3232671E1412/2025-07-11-091237.backup/2025-07-11-091237.backup/Aux

This does not work. Instead, you need to find the path using Terminal:

sudo tmutil associatedisk /Volumes/Aux /Volumes/TM\ 7/2025-07-11-091237.previous/Aux

It’s important not to have any trailing slashes. And, also, it will fail if you use -a with an APFS destination. But I guess that’s OK because there’s only one .previous folder to point it at, anyway, and APFS itself should know the chain of parent snapshots…

Previously:

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