Might look at the Pioneer HUs (Pioneer_man has been on here stating that their HUs can read an external HDD.
Any external HDD case would work for what you need. A HDD takes 12v, but I'm sure it needs to be regulated.
Just buy an external hard drive and be done with it. No need to buy a drive then an external case for it if you don't already have the drive.
USB probably wont have the needed power for the drive. IIRC, they only put out about a half a volt down the USB line by standards.