songs not appearing in Music library from external drive

My old Mac housed all of my music transferred from CDs and downloaded. The file was pretty massive and I ended up transferring a good portion of it to an external drive. I got a new Mac last year and didn't add any of the music with the intent of solely using the external drive. I've finally gotten around to making sure I can use the drive, but songs don't appear in Music automatically. I've held down the Option key while opening Music to identify the correct drive, and in Preferences I'm using the external drive as the Music Media folder location. I see the music if I open a Finder Window, and if I play a specific song from there it appears in Music. I'm sure there's a way to make the songs visible without playing every one but I'm stuck. Help!