Music library file cannot be saved, unknown error -50

Started getting this message when I use Music "The Music Library file cannont be saved. An unknown error occurred (-50).

When I next opened up Music all of my music had disappeared. (They're all saved on an external hard drive.) When I re-add files if I close Music and reopen they have disappeared again. I've set the correct file location for the external hard drive where all the files are located. Any ideas what is causing this or what this error message means? I haven't been able to find any recent discussion on this topic