Size: Easy to findLooking for a External hard drive Specs have to match the following.
Size: 300gb Or more
Power: Any thing that i can power off the car battery (Hopefully DC not AC)
Read Speed RPM: Any thing that will Stream Movies perfect.
Power: Again not really a problem
Read Spead RPM: Lets talk
RPM is how fast the disk spins and while it is used for marketing, I have seen some SCSI HD's with a much lower RPM but amuch higher data transfer rate (DTR). This is really the number you should look at and is listed on the packaging of just about every HD made. Really though, as long as you use Firewire instead of USB just about any should work fine. WalMart actually has some that will suit your needs.