"Unsupported Library" error after macOS Big Sur 11.6.1 update to Photos 6.0 (361.0.100)

Update:

One successful method of 'undoing' the damaged corrupted .photoslibrary was do a full restore of the entire Big Sur drive (to ensure that there were no other potentially corrupting updates installed) from a recent Carbon Copy Cloner copy made early Sunday. I did a full restore back onto my external Big Sur SSD. CCC took less than 90 minutes to do the full restore from a Thunderbolt 2 drive to the Thunderbolt 3-connected Big Sur drive, about 750GB total. (I was in the other room watching TV and wasn't paying close attention to time.)

Photos again opens my 500GB .photoslibrary file successfully.

It's possible that restoring ONLY the .photoslibrary file from the CCC backup might have worked, but I didn't want to chance corrupting it again and need to do yet another restore.

@TonyCollinet:

Thanks for the response.

Yes, I did see the iCloud downgrade method you linked, but I don't pay for that much iCloud storage. Yet.

I don't use iCloud/Apple ID to sync Photos on any of the Macs or iOS devices in the household, so Monterey should not have been trying to access that .photoslibrary at all.

IMO downgrading a 500GB photoslibrary via iCloud could take days or even weeks. I'm copying another 500GB .photoslibrary stored on a Mac mini media server onto a locally-attached NAS RAID1 drive over wired gigabit Ethernet and Finder estimates 2 days to completion.

Apparently because the external SSD with Big Sur and my .photoslibrary file was attached and powered-on when I booted into Monterey, even though I didn't open Photos on Monterey, nor attempt to sync with iCloud/Apple ID, nor open Finder to view or access any attached external drives, Monterey STILL corrupted the .photoslibrary file, making it eternally unusable with Big Sur.

Once 24 hours of fresh Time Machine backups and multiple CCC clones of my Big Sur drive are completed, I'll be disconnecting all external drives, and using Recovery to erase Monterey, and reinstall Big Sur onto the internal SSD. Live and learn!

Update 2:

The evil dwelling within my iMac 2019 250GB internal SSD boot drive has been purged.

Monterey 12.0.1 is banished to the dustbin of history, and a fresh copy of Big Sur 11.6.1 is now installed.

So why did I restore everything to my primary external Big Sur SSD with Carbon Copy Cloner instead of using Time Machine as I originally planned?

A Command-R Restore only reinstalls the Big Sur operating system, NOT the rest of my non-Apple software, system preferences, documents music, photos, and data.

I would need to use Migration Assistant to restore all the files from a reliable Time Machine backup.

I haven't done a full Time Machine Migration restore in years, prior to the use of APFS.

Spinning Time Machine drives are, as I understand it, still formatted as MacOS Extended Journaled.

I hadn't created or saved anything significant on the Big Sur drive in the hours between the last Carbon Copy Cloner image and the time I installed Monterey and thus eternally corrupted the important 500GB Big Sur .photoslibrary file.

So it was simpler (and perhaps faster?) to use one program (Carbon Copy Cloner) with one restore command to replace a bootable, fully working, complete set of files in one step.

This is actually the second time I've been teased by Monterey.

The first time was when an early Public Beta disabled Bluetooth, turning my iMac with factory bluetooth Magic Keyboard and bluetooth Magic Mouse into a brick until I could take it to an Apple Store. The generic USB keyboard and USB mouse I have weren't recognized. The Apple Genius used a genuine Apple USB keyboard and mouse to reboot, run diagnostics to confirm that there was not a bluetooth hardware failure, and then reinstalled Big Sur to the internal SSD.