the hard drive worked after you dropped the ipod - so i could be a connection problem. since you wrote that the hdd is connected - that could be ruled out.
there are surely a few things you could try:
programs like "recover my ipod" are one thing you could try
but the first thing i would try is the "ear and finger test" ;-)
can you hear the hard drive spin when you power on the ipod?
or in case you are older and can't hear that much (like me - lol) - can you feel the vibrations of the drive?
when you disassembled the ipod - did you place the hdd near a speaker, a megnetic screwdriver or something else that could be magnetic?