Photos library to external drive worked and then failed

First, how is your external drive formatted? 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 drives 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

If the format is OK, then I don't understand,

"migrated my Photos library to an external drive."

What does "migrated" mean? If it were me, I'd just drag the Library to the external drive. What did you do?

I don't understand,

"I went into the Photos application settings and switched it back to the "System Photo Library" which should be the Photos library on my external drive."

I'm not sure what "it" is that you switched.

Let us know…

Noah Wallach wrote: … Ah so the external drive cannot be a FreeNAS system mounted via samba?

No. As you've seen, that doesn't work. The external drive can't be networked or in the cloud, or anything but directly wired to the computer.

Definitely not. A NAS is a headless computer running its own operating system and you'e trying to operate a Mac database from there. See the problem?

Edit: for some reason Richard's posts are only appearing after I reply. Sorry for the duplication.

What format is the external drive? It needs to be formatted Mac OS X Extended, Journaled or apfs.