Anyone try this? I'm thing if I take a molex adapter and splice it to a cigarette adapter, it should power it. Then just get a IDE to USB adapter and I'm good to go. Thoughts?
No. You're going to need a 12V (11-14V) to 12V
+10% and 12V to 5V
+10%, and I wouldn't feel safe with that unless they shared grounds. Then, you'd have to get the HDD and the adapter securely mounted.
Your best and simplest bet will be to just buy a 2.5" (5V only) USB HDD, and a USB car adapter for it to get power from (a cell phone charger that can supply more than 500mA/port would be ideal, so one of the plugs can be power, and the other data). They're big enough for large collections of lossy compressed music, these days.
An HDD will be fin in the car, as long as it is well-mounted. If you have to use the glove box, velcro tape should be fine (if you get a USB drive, get one with one of the large sides flat). I can speak from experience on this one, with whole desktop computers running in cars. I won't say any HDD will last as long as if mounted to a sturdy steel case that never moves, but if your ride isn't jarring, it won't be too much for your HDD to handle.