Playlists don't work

[Trovacaustica]

Here's the situation:

All my music is stored in an external disc connected via USB. But, everytime i disconnect the disc and/or change the USB port, my playlists don't work anymore, even (sometimes) when I connect back the disc to the original port. The playlists show the items, but they don't work because apparently the directories are changed. Is there a way to prevent this? Or, if not, is there an easy way to, say, redirect the directories or edit the playlists so they work again?