Photos and Big Sur Update

Are all the external drives correctly formatted for a photos library?

They should be APFS, or MacOS extended (journaled), with the "Ignore ownership" checkbox checked. They should not have been used for time machine backups.

You may need to give it time. It is also possible that shutting down the machine, and/or running first aid may have restarted the process, so it may not have been running for as long as you think.

Double check also that the USB port you are plugged into is USB 3 and not USB2.

As an alternative, you could plug the drive into one of the machines that has already successfully updated its library, and open the library from that photos to see if it runs any quicker on the other machines. (Assuming all machines are on the same version of Big Sur.

What kind of Macs are you using? iMacs or portable Macs? On a pietable Mac ghe loop of “analysing scenes” etc will be paused, as soon as the mac is running on battery power. It will depend on the size of the library how long it will take. For my main library with 50000 photos it took several months to complete and it will start over again, whenever you import new photos or add adjustments and metadata to Photos. The analysis may hang, if you have videos or photos in your library, that Photos 6 on Big Sur can no longer process.

I have not yet used Caffein on Big Sur, so I don't know, if the most recent version has been updated to work well with Big Sur’s energy saver and battery management. Has the version you are using been designed for Big Sur? If not, try also without a third party app in the loop. On my Mac it turned out the older videos snd dome Live Photos created by third party apps gave ben blocking the analysis of the Library and made it hang at 90%. I tested the videos snd Live Photis by importing them in small groups to a new library. If the curation cojld be completed in the new library, I left thrr er m in the old library. But if the videos could not be analyzed in the new library, I removed them from the old library and replaced them by a converted version.