Hi igouana,
Oh, sorry, I misread your post.
Have you tried formatting your player using the MP3 Player Utilities? Have a look at this post here:
How To: Removing Memory Hack. It will guid you through the process.
Hopefully you'll be able to get the rest back.
You also try formatting it as FAT as well though it may not wor as it's 8GB.
Let us know of the result.