it depends on the mp3 player.
i had a similar problem, and managed to fix it by formatting the player through windows while connected through USB.
connect player to PC,
in "my computer", right click your mp3 player,
select format
format the mp3 as "fat" or "fat32" (these are ok, but other options will damage disk)
hope this works
Paul Stobbart stoby@btinternet.com, May 2007