There will be a line somewhere in the car that is powered while the car is cranking and when it is finally running. The ECU and other computers need such power. Get a service manual, and you will probably find several places where you can connect your HD. If necessary, use a relay to switch a direct connection from the battery.
London, Ontario, Canada
Player: Cyrix 200, 32MB RAM, 10.2Gig Quantum HD, Onboard EtherNet/Sound/Video, Custom Lexan Case, Arise DC-DC, Win95 Kernal w/Custom Player
Car: '86 Mazda RX-7 w/Basic Performance Upgrades