If this always happens exactly as you're describing it (in other words, the failure never ever ever happens indoors, but it sometimes happens in the car), then the most likely problem is voltage.

The disk drives require more electricity to power on and spin up than they do for normal use. When you first power on the player, it draws about 1000ma at 12 volts. If the main power wire (yellow) to the player isn't able to supply quite that much, you would very likely get a hard disk not found error.

Check the wiring, check all fuses, especially the inline fuse on the yellow wire. If your connections are crimped, solder them instead. Check for the wires pulled out problem. Check to make sure your car's battery, charging system, and voltage regulator are doing the right things and supplying enough power. Check to make sure you haven't wired the amplifiers wrong, such as wiring the amplifiers to the dash harness (amps should be run directly to the battery with heavy gauge wire).

If, on the other hand, you still occasionally get Hard Disk Not Found message when you're in the house, then forget what I said above. It's one of the usual problems, and the reason it's coming up mostly in the car is temperature differences, thermal cycling causing components to expand and contract, making cracked solder joints or badly crimped cables lose their connection. Or, possibly, extreme temperatures making the disk drive fail or perhaps hit its thermal cutoff.