Ok, j, you've got the pictures in a folder on your computer, but for some reason they don't import. First, use Disk Utility to make sure that the external drive is formatted as either APFS format or Mac OS Extended (Journaled). In newer versions, Photos is way more dependent on the format of the drive, and troubles may not show up immediately. And it's also important that, since formatting, Time Machine has not been on the drive. A Time Machine drive allows only Time Machine to be on it.
How are the pictures formatted? jpg, Raw, png, or what? Can you view an image in Finder by selecting it and pressing spacebar (Quickview)? Can you drag one of these contrary image files into Preview?
How are your Photos preferences set?
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" checked?
Have you tried 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.