I have the same issue. My Zen Stone is in this exact condition after I reformatted it. The reason I reformatted it was that I had removed without unmounting and thereby screwed up the filesystem. It has some issue with a bad superblock so I had to mount using the -o loop option. I'm guessing that the bad superblock is the source of the problem but unfortunately fsck believes all is peachy keen and has nothing to say on the matter.
Btw. mine is green, but the color is completely irrelevant. I can tell you that much. ;)
Rob Levy r.p.levy@gmail.com, September 2008