when you put it in disk mode, are you able to access the hdd from your computer?
try running some chkdsk tool over the harddisk or try to recover with ipod-updater
if there's a tickering noise coming from the harddisk it most likely is broken, but as you can read at several places you can make it work for some time again when you smash it on the left or right edge.
once i have bought a defective ipod 4g u2 edition in ebay which should have a broken hdd, but arrived working. I assume the ipod was transported quite rough, so that the head, that was locked somehow got "free" again.
now the player works most of the time, though i have to do the "smash" from time to time. I know the hdd is bad, maybe soon i have money for a new one.