I had a similar problem with my Wewa 256MB mp3 player.  I deleted all the files on the disk but my computer still said it was full, so I formatted the drive.  Then I got the error message: "Error! Reformat drive".
Right-click on the removable drive (your mp3 player) and choose 'format' and then under 'file system' choose FAT and do not select 'quick format'.  This worked for me, and I was able to load files onto it and play mp3s again.  Hope this helps.
                
 Warren
                
 September 2007