Need help with Sony mp4 flash drive

I’d bought a sony mp4 flash drive. It worked fine and showed 998mb of memory space for a few days. But when i formated it by the given cd’s software, i’s showing just 475mb of memory space. Then i format it many times to recover my previous memory space. But i failed. Where’s gone rest of my memory space? What should i do to solve this problem? If anybody know the solution, please informe me.
My e.mail address is "ss_jalil@yahoo.com".