Exactly as has been said, the only harddrives that will run off the 500mA power supplied by a single usb are 1.8" notebook ones, even 2.5" drives require twice that. After that you're depending on the fact that the headunit has firmware that's able to cope with that many gigabytes, and you have to use their interface to access your library of songs. Sounds to me like 2 substantial hurdles, my advice is for something cheap and effective is to get a hu with an aux. input and work an mp3 player through that.