It's equally amazing that you don't understand that every commercially available DVR made works that way.
1.You best take a closer look at the table in the DVR comparison thread;
2. They don't "all" run this way,
3. The ones that do, don't, or shouldn't have to,
4. If the HDD is not is use and won't be for a extended period of time, there is no practical reason it should be running. Period!
5. Familiarize yourself with the operation of a hybrid vehicle, the engine only runs when it needs to why should it be any different here?
What part of the above don't you