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This issue has been solved. I had to completely repartition and format the external disks and ensure they contained at least one APFS Container. I’m guessing there is permission metadata contained in this that isn’t available with the partitions created in Catalina.
Once they were partitioned and formatted in BigSur, and contained a container I was able to drag and drop my Photos library folder across and then right click and open Photos with this Library. Once inside I could set this as the System Library.
Moving from Mojave to BigSur, skipping Catalina I hit a snag with an external Photos Library. I’ve kept all my digital photos since the beginning in iPhotos and then in Photos. The Library got big enough (700GB) that I moved it off onto its own external drive. This was a dedicated 1TB SSD that was on my MacPro 5,1.
I’m moving over to MacPro 6,1 (yes, a Trashcan, I won’t be buying a 7,1 – I will probably wait for a M1 MacPro in a couple of years.) and so I moved my Library over to a NvME disk and am using it in an Thunderbolt Enclosure with the new MP6,1. Apparently however, BigSur doesn’t like the Library on an external drive. Maybe it knows it’s ejectable? Not sure. Trying to start Photos begins the Upgrade process but it always fails.
I got one hit that said that adding Photos to Full Disk Access could solve this.
This however didn’t solve the issue.
I had space on my boot drive so I was able to copy the Library over and Photos did open it and upgrade it on the boot Volume. So it’s definitely something about the new disk images and security and access in Cat and now in BigSur. Eventually I’m going to have to move it another drive or get a 2TB boot drive and upgrade by MacPro.
One last thing I’ll try is to see if I can make the Mac think the thunderbolt NVME drive enclosure is _not_ removable and then that may change the access patterns of apps like Photos. Clearly, you can’t have your drive ejecting in the middle of Photos being open and Apple is probably marking these for that reason. If something works I’ll update this post with the new information.