Time Machine snapshot could not be created for the disk "%@"

Hi. Firstly, thank you for the reply. It's great to know that this new Big Sur upgrade issue is not unique to me! :)

After much searching and trying different things, I finally gave up on finding a quick solution (no doubt there will be one).

I also reviewed a few of the 3rd Party Backup solutions, thinking of just ditching Time Machine.

But in the end I took the more brute force approach of a clean install of Big Sur.

To achieve this (without my Time Machine backup to recover from), I grabbed a spare 2TB USB drive that I had, and also a 16GB SD Card.

I then created a Big Sur installer disk on the SD Card, and clean formatted the USB Drive.

I then booted from the SD Card and created a clean Big Sur installation on the external USB Drive, which included Migrating all my data across from the internal Machintosh HD - Data drive.

Once complete this gave me the confidence of a full back-up, on an external drive, that I could boot-up from (if all else failed).

However, when the migration was successfully complete, instead of signing in to my replicated external boot drive, I instead shutdown and again booted into the SD Card Big Sur installation disk.

I then proceeded to completely erase my internal 1.12TB Fusion Drive (note this also required me to then recombine the resulting seperated / formatted 1TB disk0 drive and 121GB disk1 SSD drive, to re-establist the combined 1.12TB empty Fusion drive - an easy terminal diskutil command).

Finally, as I first did above with the External drive, I now created a clean Big Sur installation on the cleanly formatted internal 1.12TB Fusion Drive, specifying to migrate my data back from my attached USB Drive.

Once this was all completed, I shutdown and removed the SD Card & USB Drive, then rebooted and signed back into my newly installed and recovered internal Fusion Drive, re-signed back into iCloud, and after a short while of disk activity I have everything back and a seemingly snapier machine! :) (Note: This would be my first "clean OS install" since continuously upgrading the OS since I bought the iMac new in late 2014).

Of course, the final step was then re-activating Time Machine, to again start backing-up to my also cleanly erased Time Capsule. This all now worked flawlessly. So, original problem solved! :)

Note that with a nearly full 1.12TB Fusion Drive this process took about 8 hours to create the replicated USB3 connected bootable system Drive, but only around 4 hours to then re-create and replicate everything back to the Internal Fusion Drive.

In summary, although I have no idea what the cause was (or a more direct fix is), for the original "Time Machine could not back up the disk "%@" issue, I'm actually very happy with the way I resolved this. Because I now have the satisfaction of a cleanly installed Big Sur OS, with all of my accounts and data restored and a snapier "cleaner feeling" system.

But of course, you need to make your own choices on how to resolve your own issue, as (of course) YMMV.

Hope this is helpful.

For what it's worth, although I didn't mention it in my original post, I also had skipped Catalina, and was upgrading directly from Mojave to Big Sur. So this does appear to be a common cause!

Also, regarding the above comment: "Disk Utility First Aid says external drive if fine. Just to be sure I erased and reformatted it."

Note that the reported error is during the creation of the disk snapshot, which I understand is before data is written to your external Time Machine backup drive. Therefore, applying Disk Utility First Aid to your external drive and/or reformatting it, is not where the problem lies. The problem is with the creation of a snapshot of, and on, your internal Mac OS drive.

Hi Tim. I also upgraded to 11.1 yesterday, and I can report that my TM Backups are still working fine. However, in my case I had resolved the original issue via a clean Bug Sur install. I was hopeful that Apple had fixed the original Big Sur upgrade TM issue with this release.