Upgraded to Big Sur - is Time Machine backup usable?

After a long wait for the dust to settle I finally upgraded from High Sierra to Big Sur.
In the past, I once successfully booted from a CCC clone, and when I booted again from the internal drive my TimeMachine backups were not accessible anymore. TimeMachine seems to have regarded the Mac as a new machine.
I couldn’t find any clues how to continue using the TM backups and had to start with a new TM history.

Now with having upgraded to Big Sur I am afraid to connect the TM backup drive, because I want to keep the incremental backups.

  1. Are these backups made in High Sierra compatible with Big Sur?
  2. If so, how can I avoid losing access to the old backups?

I have spent plenty of time searching the init and read through Take Control Books, but couldn’t find anything. Maybe I overlooked something somewhere ?

What would you do?


There’s a way to inherit time machine backups. A quick google found:


While I have done this procedure with previous versions of macOS (Mojave I think is the latest) and it worked just fine, I haven’t on Catalina or Big Sur.


You could clone your Time Machine backup first as a precaution. According to this article, Carbon Copy Cloner cannot but Superduper can clone a Time Machine Backup even in free mode. The author wanted to move his Time Machine backup to a bigger hard drive:

According to another article, Time Machine backups on Big Sur can use the new Apple disk format APFS which the author is excited about.

At home, I am stuck at Catalina and I haven’t upgraded my wife’s work computers yet, so I don’t have Big Sur experience. I am wondering if Time Machine will just continue using your backup with its current format.

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In the comments to

I found this which suggests that Bur Sur will keep backing up without requiring a reformat to APFS:

artiste212 on December 8, 2020 at 2:12 pm
I bought a new M1 Mini and asked TM to continue using the TM disk from my old, 2012 Mini that ran Catalina. There were never any problems backing up to the TM under Catalina and, apart from 2 hours spend “preparing to backup” with the new machine, no problems under Big Sur. It didn’t ask me to update the partition to APFS.

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I updated to Big Sur and had no problems with TM. I started the update just before bedtime and went to sleep. Ah, a fresh face in the AM.

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Thanks for all the information and comments.
I plugged in the backup drive and Big Sur started to add backups to the drive.
What I don’t get is that in Time Machine I can go back to the backups of 21Jun21, which is when I switched to Big Sur, but I can’t access the older backups going back to April 21, although they are displayed in the TM screen as well as in the Finder. (I attach screen shots).
I wonder why that is. :thinking:
The link posted by blm https://acecilia.medium.com/mac-and-time-machine-inherit-a-backup-from-another-computer-dbad4d8aad79 is quite interesting, but I’m not sure whether this applies as Big Sur has already added backups to the disk.
TimeMachine snippet

It should. All associatedisk does is tell Time Machine to use a different destination for its backups. Any other destinations laying around won’t be used, so the new backup starting June 21 will just be ignored, and could be deleted at some point, once you’re sure the backup to the old destination is working.

As I said, I haven’t personally tried this in Big Sur, but I have in an earlier OS, and it worked as described.