Solid state drives like the poster above, was my suggestion. However, they are VERY pricey at the moment. Give it time.

A carputer would be your best bet in terms of options, adaptability which will give the choice longevity. I agree with nickm50, the only current solution that will withstand the sands of time, is a full PC in the car. This is a project i'm looking heavily into at the moment as well.

Disk based storage is susceptible (especially HD) to temperature, vibration and other such variables that were not designed for mobile use. Laptop drives are a huge leap from standard desktop drives, but are still not immune from the elements. This is where solid state drives come to play, but as stated above, are a little to "new" to the market for pricing to drop drastically.

www.mp3car.com/vbulletin is a site just dedicated to the niche. All sorts of info, including answers to your question are most likely on that site.

Hope this helps!