It is hooked up to a USB battery pack, which in turn is plugged into the accessory port on the car. The hard drive powers down after the data transfer is finished.
All data is encrypted before sending to it. If it gets stolen, well it is only one of the copies of the data (however it lives hidden under the dash, and is small enough that it looks like a piece of car equipment).
I've never specifically used a Li-Ion pack in my car PC setup, storing it in the heat would worry me a bit, but surprisingly, operational temps in a car aren't bad: If he's using it after he drives it, the car has likely been cooled or heated for driver comfort, and it'll take time after he parks for the temperature in the car to return to "outdoor" levels.
My bigger concern is just storing it. It sounds like he’s using just a regular powerbank type battery which is li-ion. Letting it sit in a parking lot for a day in the summer then turning it on probably isn’t a very good thermal situation. I wonder if some sort of sealed lead acid might be better. Far more inconvenient for power management though as some sort of circuit would have to be created.
All data is encrypted before sending to it. If it gets stolen, well it is only one of the copies of the data (however it lives hidden under the dash, and is small enough that it looks like a piece of car equipment).