Music files become unavailable to the Music app after I migrate them to an external drive

Monterey, an older 27-inch iMac.

I moved my whole music library to an external drive. When I try to play a track from that drive, I get the message "could not be used because the original file could not be found". Obviously the first thing I did in Music was to change the place the application looks for files. Change was accepted, but Music still throws up that message every time, even though Get Info reports that the file location data is correct.

Also, another user has posted an AppleScript that can be used to change the data for any files where it didn't take. Thank you, Steve; looks great. However, having installed the script where it's supposed to go, Music was supposed to gain another menu in the menu bar so you can run scripts. Didn't work.

Is there some black art to get this stuff to work? TIA.