I have a 1GB MP3 player, but can only use 500MB

Hello. I have browsed through the questions that were answered on this site, but did not seem to find the problem that I'm experiencing. One fine day my 1GB MP3 player told me it can only use half of its volume. It was perfectly alright until then, I could easily store a whole movie on it. I tried scanning, fixing the errors and even thought about reformatting, however a little research on the web convinced me this will do more harm than good and then the player's manual warned me against reformatting. Does anyone have a suggestion on how to fix it, it will be appreciated.