Mac Photos App: stuck at "Updating Library 94% complete"

I have the exact same problem, upgrading from Catalina... it's just sitting at 94% and using no CPU.... The drive in question was used in Catalina for my photos library and it has 180 gb of storage available on it. There is NOT enough free space on the volume to make a copy of the library. It's a 2TB drive. Any suggestions appreciated.

Is your external drive compatible with Photos? Is it a locally mounted drive, formatted APFS, not case-sensitive, and has never been used for Time Machine backups as described here (Move your Photos library to save space on your Mac - Apple Support)? Is it possible to set the "Ignore ownership on this volume" flag? And do you have plenty of free storage on the external volume? I am keeping enough free storage on my external volumes to be able to store complete copy of the library as working space when the library is repaired or upgraded.

If the external drive is not suitable or is full, copy your library to a different volume and try again to upgrade it. And do not interrupt the upgrade, even if it takes hours. My Library is roughly the same size as yours, and it took half the day to upgrade. If the library cannot be upgraded, restore it from a backup you made, before you upgraded to the current system version.

Which system version did you upgrade from? Is your library syncing with iCloud Photos or a local library?