I have an external disk that has Time Machine backups from three different Macs. How can I delete the backup set for one Mac without affecting the other backup sets? Thanks.
You can just drag the folder to the trash and empty it.
BUT
It will take hours to complete as it traces all the linked files. The only quick way to do it is to reformat the drive. 
Dave
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I had a similar problem when my wife bought a new Macbook Air and we transferred her part of a 2017 Air we had shared to her new one, making her the prime user. We decided the simplest thing was to set up separate new Time Machines for both of us and save the previous one as a secondary backup. We had a couple of spare hard drives available so it wasn’t worth fiddling around a lot.
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Oh, joy. Now I need to decide if I want to save the other backups.
You could buy a new drive . . .
How old is the current one?
Dave
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I have two SSDs on order. I thought they would have been here by now.
The current drive (spinning platter) is over ten years old, but it’s been in use for under five years. (It was free and I didn’t have a use for it and forgot I had it.)
Select Empty Trash and go have a cocktail…easy peasy.
Well, first I would need to get the backup into the Trash, and that didn’t work. (Allegedly, some program was using some file.) After trying a few times, I took @Dafuki’s suggestion and reformatted the drive.
(This may be out of date—see below*)
It’s been a while since I had to do this as TimeMachine on my TimeCapsule is so unreliable, and I now use an external disk that I plug in to do TM backups. But I did discover a way to use a Terminal command to deleting one machine’s TM backup whilst preserving the other Macs’ TM backups on the same disk.
First mount your TimeMachine disk (be it in a TimeCapsule or on a connected external disk). Open Terminal and change directory to that disk:
cd /<drag disk icon into Terminal window>
Check the backup you want to delete is visible there:
ls
Issue the command to delete the sparsebundle:
$ rm -Rfv <drag the sparsebundle to the Terminal window>
When you press Return the individual bands inside the sparsebundle will be deleted one by one. This takes some time, depending on how long the backup has been building, but is MUCH quicker than trying to Trash it and empty the Trash. It is far quicker to reformat the disk, but the advantage of doing it this way is that you don’t destroy the TM backups of the other Macs.
*I looked at my external SSD that I use for TM, and find it has individual folders for each backup, but it only backs up a single Mac. Are sparsebundles now gone, or are they only created when more than one Mac backs up to a TM disk?
Thanks for the procedure for deleting using rm. As noted above, I reformatted the drive.
Based on what I saw when I tried to delete a backup set (and not based on any real knowledge), I would guess that your disk is formatted as HFS+ rather than APFS. Corrections, anyone?
Sparsebundles are used for remote/network TM volumes. Their primary purpose is to make sure Mac-specific file system features won’t get lost when a backup is saved to a file server that might not be using a Mac file system.
When backing up to local storage, a different mechanism is used.
Time Machine volumes created since Big Sur (macOS 11) are APFS volumes. All the backed-up files are copied to folders on the volume. Each time, a snapshot is generated, which is the mechanism used to track the version history and prevent duplication of data that has not changed. If you browse the volume, you will see a snapshot for each backup. If you open a snapshot, you will see the state of the backed-up data as it was at the time of that snapshot.
I’m not sure how Time Machine-on-APFS (aka TMA) handles multiple volumes or multiple computers backed up to the same destination. I think you’re expected to create a new APFS volume within the same APFS container (which will therefore share free space with the all other volumes in the containers) and configure each computer to back-up to a different such volume. This way snapshots of one computer’s backup won’t affect another computer’s backups. But I don’t have personal experience with this.
Time Machine volumes created on Catalina (macOS 10.15) or older are HFS+ volumes. All the backed-up files are copied to folders on the volume. Each backup is a folder containing the entire contents of that backup. Hard links (of both files and directories) are used to “deduplicate” data, so files that haven’t changed won’t waste space on the backup device.
If you back-up multiple volumes or computers to a single Time Machine-on-HFS+ (TMH) device, each computer/volume gets its own directory, containing all of the backups for that volume.
When backing up to a remote/network device, Time Machine creates a sparsebundle disk image. The image is formatted HFS+ or APFS (depending on the version of macOS that created it) and (I think) the content closely resembles what is stored on a locally-attached Time Machine volume.
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