Time Machine not woriking with Big Sur 11.1

Are you running Sophos anti-virus? If so, you need to do one of two things. 1. Uninstall Sophos until January 2021. (Sophos is promising a fix by then) 2. Continue to run Sophos and disable time-machine until there's a fix.

If you do not have Sophos then disregard this suggestion.

Big Sur is not compatible with older TM backups. Is the external drive dedicated to TM? You may have to reformat the drive to APFS and start over with the TM backups.

Well I said it may have. It should have asked you if you wanted to reformat it. TM in Big Sur prefers APFS format. And Big Sur TM is not compatible with previous OS versions of TM. Best thing to have done was to put previous TM backups elsewhere and start fresh in Big Sur.

It doesn't matter what the disk was formatted as before. TM could have possibly reformatted it was my point. I just meant to look to be sure.

That changed with Big Sur. Time machine will reformat the backup disk to APFS if it isn't already formatted to that.

Well I have no idea what going on, unless the drive needs repair. I'd use Disk Utility to verify the drive and repair it necessary. As I said, my be best to erase the drive and start over completely if troubles continue.

Not true. My TM is formatted from Mojave use (Mac OS Journaled) and works fine.

What did you do to get it to work the first time? Is it a continuation of where your last OS, or did you start a new backup? Possibly, the OS doesn't like the drive you are using. If you have other backups, I'd completely erase the drive and start over. Or try another external to backup to.

I am not an expert, but.....the Apple documents regarding Time Machine state:

"Time Machine can’t back up to an APFS-formatted disk."

This is specifically referring to the target backup disk, i.e., the disk that Time Machine will use for the backup. It is not referring to the source disk, i.e., the disk that Big Sur is on, which is APFS.

Generally the Time Machine disk should be formatted to Mac OS Extended (journaled).

Again, I am not an expert. But just bought a SSD for backup purposes and ran across that tidbit prior to setting it up. I am assuming that Apple is referring to any disk type, hard drive or SSD with that statement.

Big Sur may have reformatted that drive to APFS. Check it and see.