Well, from my usb memory stick the head unit does get confuesed sometimes with only 200 songs. A harddrive needs more power than a memory stick, as its not flash memory its a disc being written onto, Normally a external HDD has a powersupply to power in the wall as far as the ones i have. So to be honest, im not sure if it would work, and if it did, you would want to have the top dollar head unit or i think it would just get confused and lagg. Plus im not sure it could power it, unless it only runs of the usb, and not another poer supply. But then when u are transfering songs, power will be less, and well
I just dont know but i wouldnt advise it. lol