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On my iMac I have a dual boot setup with currently Big Sur and Mojave (for those 32 bit apps that probably never will be updated and that I can't really live without.) I also have a 3rd partition with shared data like documents.

I want to make a Time Machine backup of the whole computer, but can I have a complete Time Machine backup from both OS:es to the same target? Currently I have one TM backup for Mojave+Shared running from Mojave and one backup for Big Sur+Shared running from Big Sur. This causes the shared partition to be backed up twice, wasting lots of space on the backup server, so I would really like to get away from this. The solution of backing up the shared partition from only one of the places I don't want to do since I generally don't switch often between the OS:es, which then can make the shared partiton backup old until it gets updated. Is this possible?

Situation now Desired

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  • I highly doubt it but am not 100%. The problem I think is that your Mojave and BigSur installation would have to share the backup process and work files, i.e. Mojave would need to know where you left off on BigSur and vice versa. Apart from this not being possible TM is not always downward/upward compatible...
    – X_841
    Commented Dec 14, 2021 at 11:56
  • Doubtful. TimeMachine is an intentionally simple backup system, and you are trying to do something that is "not simple." kind of like pouring coke and maple syrup into the same glass and expecting them to not mix. Better would be something like SuperDuper where you can have one backup for "common" files and one for each OS partition. Commented Dec 14, 2021 at 13:23

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