Photos Library » (3146).

One thing that we find a lot is that the external disk may not be formatted in a way that is friendly to Photos.

The external drive must be formatted in either APFS format or Mac OS Extended (Journaled) format. And the external drive must not have had Time Machine on it since it was formatted. See this:

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

This seems the most likely problem. But if formatting isn't the problem, try these things:

Restart the computer (of course)

• Rebuild your Photos Library--close Photos and re-launch by option-command-clicking the app icon.
• Make a new user and see if the same problem recurs with it
• Use 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.
• Start in Safe Mode (different for different computers) This bypasses certain potentially disruptive processes.

           Start up your Mac in safe mode - Apple Support 

One of these steps may fix the problem, but their failure to work is also diagnostic.

If you still need help, get back to us with this info:

In Photos>Preferences>General,

Is "Use as System Photo Library" grayed out (indicating it is) or solid (indicating this isn't your system library)?

Is "Copy Item to the Photos Library" checked? (This is the default, indicating a "Managed Library")

In Photos>Preferences>iCloud

Is "iCloud Photos" checked?

If it is, is "Optimize Mac Storage" check?