Just to close the loop on this:
I changed GM back to internal storage,
Renamed /extSdCard/Android/data/com.google.android.music to /extSdCard/Android/data/com.google.android.music_orig.
Tried to change GM back to external, and got a FC
At that point the option to switch was greyed out
I rebooted the phone, was able to switch to external and now it works like it's supposed to again.
So something about the external storage location must have been corrupted. I think it may stem from using the Music2SD Xposed module before GM was updated to support external.
Anyway, all fixed!