Apple Music not showing songs/albums on new M1

Hi there.

Just upgraded to an Apple Mac mini. But having big issues with Music.

I am running my media off an external drive. I moved all my old stuff onto the drive when moving from my iMac (iTunes) then followed the apple instructions: used the old iTunes.itl file to kick start Music on Big Sur, after consolidating all my files.

It works fine for a day or so and then all my songs disappear. They cannot be viewed. Weird thing is; all the playlists are intact and all the music in them plays fine (it's usually the other way round when moving to Music). So the music is there; but it just can't be viewed. All the permissions are fine (made sure I can read/write etc and nothing is locked) and I have granted full disk access in Security.

I even wiped my hard drive and started afresh (other issues), but it had no affect.

I just can't figure this out, particularly as it works for a while; then drops off. I could start with a new library and re-import, but I don't really want to lose all the tags and data I have stored.