Hundreds (if not thousands of people are having this problem with RCA MP3 players that use hitachi microdrives. The only option you have is to get RCA to replace it.
If you only want to use it as an external memory, you can call Hitachi and get them to replace the bad hard drive.
More info here:
http://jackwhispers.blogspot.com/2007_05_01_archive.html
I'm working on getting the proper file system from RCA so I can repair mine and possibly others broken RCA HDD MP3 players.
Matt
December 2007