Mmmm... have a look at /var/log/mpd/mpd.log (via ftp client or using "sudo nano /var/log/mpd/mpd.log" from a terminal session) and see if there is anything in there that gives away any clues. Every time something is added to or removed from the library it is logged there, including error messages when an add/remove fails... something in there may point the way (he says hopefully)...