Retrieving Photos

What format is the external drive? It has always been necessary to store photos libraries on a macos extended, or APFS formatted drive in order to avoid data loss. However, later versions of MacOS have become more sensitive to this.

Check by selecting the drive in finder and hitting CMD-i

How is the external hard drive connected to your Mac?

Try to plug it directly in, if you have been accessing it over the network. In addition to the file system format pointed out by Tony, an external volume needs a wired connection and must not have been used for Time Machine backups or have any backup software installed, if you want to use it with Photos.

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