slight typo in weazel's post...he meant "chkdsk /f".
I'm not sure if the jb3s operate as a removable drive, but if they do it should work the same way.
If that is the case, just get to a command prompt (on a windows machine just click 'start' then 'run' type 'cmd' and click 'ok'.
Then change to the drive that your iriver (errr...jb3) is on...and type "chkdsk /f" and it may recover the files...