When you attach a USB HDD with music, the database for this disk is being created.
Judging by the settings in mpd.conf, DB file is created in the system itself, not on a connected USB disk:
# Volumio MPD Configuration File
# Files and directories
music_directory "/var/lib/mpd/music"
playlist_directory "/var/lib/mpd/playlists"
db_file "/var/lib/mpd/tag_cache"
If you disconnect this disk and attach another disk, the database will be created again, and the database of the previous disk will be erased. This is very wrong. What would it avoid,you must make that the database is created on the disk itself. Tell me how to do it?