This error message means that you did not remember to read-only mount (ro, rom) the player after you messed around on the hard disk. Don't forget to do this!

From an old thread where Brian Mihulka was answering the same question when I did the same thing, the solution is to do:

fsck.ext2 /dev/hda5

I would like to put this in the FAQ. For those who know: Is this still the correct thing to do?