Time Machine disk used for multiple computers

I think I have done this before, and others have said it's possible to back up two computers to the same disk, but after recently reformatting my Time Machine disk I am encountering the error

<disk name> contains backups created on a different computer

At which point I am asked if I want to claim these other backups (I don't, not on this second device) or to Start New Backup. If I choose to start a new backup, I'm asked to encrypt the disk which is already encrypted. This feels like it will wipe the existing backups off the disk, or else how will it manage two different passwords for two separately encrypted directories on the same disk/partition?

The Time Machine disk is APFS case-sensitive encrypted. The existing backup is from macOS Ventura; the one I'm trying to add to it is Big Sur.

Earlier Mac models

Posted on Feb 28, 2024 2:31 PM

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John Galt

Level 10

160,120 points

Posted on Feb 28, 2024 2:48 PM

If you choose Start New Backup, it will not affect backups of other Macs, or the same Mac that may have backed up to that disk in the past. Time Machine keeps track of the passwords.

The obvious disadvantage is that the new backups will be limited by the remaining available space, and there is a possibility it might not be sufficient even for one backup. Time Machine expects to use the entire capacity of the backup drive. Upon reaching that limit it begins to delete older backups. Given the fact some of that capacity has already been allocated, you will reach that point earlier than you otherwise would.

Other than that there are no concerns.

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Time Machine disk used for multiple computers

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