Photo 9.0 repair library

Paul Korntheuer wrote: The Photo library has worked fine on this drive for years

Unfortunately, that doesn't mean nothing was wrong with it. I'd try to copy the Library to another hard drive and see if it works from there.

The usual things to try first include

• Restart the computer (of course)
• Start in Safe Mode (different for different computers) This bypasses certain potentially disruptive processes.

Start up your Mac in safe mode - Apple Support 

• Make a new user and see if the same problem recurs with it. This tells you if the account has a problem
• Try another library if you have one, or make a small test Library with a few pictures in it and see if the same thing happens.
• Try again to rebuild your Photos Library by closing Photos and re-launching by option-command-clicking the app icon.

Can you use the Library at all? This could be a corrupt file that's getting caught in the gears, somehow. If you can use the Library, you can go though and see if there are pictures or videos that won't load or edit. Showing up as a thumbnail but not allowing editing is telling. Videos are often the culpret -- their codecs change without warning.

I don't understand the problem with creating a new Library. For making a small test Library, use your internal drive.

Let us know what happens...

Paul Korntheuer wrote my Photos library on an external hard drive,

The first thing we need to know is, How is your external drive formatted. We always ask because, to avoid damaging the Photos Library an external drive must be formatted in either APFS format or Mac OS Extended (Journaled) format. Additionally, the drive can not have had Time Machine on it since it was formatted. There have been so many problems with using incompatible drive formats that the newest macOSs won't even allow a Library on a non-Mac formatted drive to open, since there is a chance of damaging the Photos database. See this:

Move your Photos library to save space on your Mac - Apple Support

Great! Be sure to format SSDs with APSF.

I've had good luck with Crucial. This one is about the size of maybe 2 flash drives. $243 on Amazon

Crucial X9 4TB Portable SSD - Up to 1050MB/s Read - PC and Mac, Lightweight and Small 

This one is pretty cute, but 2 TB seems the largest, and it's $150. It's a bit smaller than the above one, but a little fatter. What's impressive is the transfer speed, twice the above. But I don't have a port on my M1 Air to support it. I'm planning on upgrading...

SSK Portable SSD 2TB, up to 2000MB/s External Solid State Drives, USB 3.2 Gen2X2 NVMe SSD External Hard Drive, Ultra Low Power

They both weigh about 1 oz. Amazing, really.