A laptop with motion sensing likely won't save an external drive. The external will connnect thru USB or Firewire and I doubt the laptops will send a drive park command over those interfaces when the motion detection trips.
The external drive is unlikely to have motion sensing. It's posible but most don't have it.
Depending on how you mount it... The vibration of driving, even on fairly rough roads, shouldn't hurt it. You'd have to crash the car to hit it hard enough to crash the drive. Even the most basic shock mounting, or just leaving it on a seat, should be enough protection. (Go read modern HD specs for G load in operation. It's allot higher than it used to be for most drives.)
You can also buy larger HD for the laptop itself. The laptop drives are all standard but you'll probably have to change some brackets and maybe plugs. It can be a pain on some models but most are fairly easy to change. You can also use Micro Drives and Flash cards in PC Card slots. (You'll need an adapter to use Compact Flash/MD in PCcard slots.) Some newer laptops also have SD and other slots besides the PCcard/PCMCIA slot(s). Micro Drives are really small HD that are ment for cameras and other things that can see allot of banging around.
In reality... Do you really need your entire collection in a car? How far can you drive in a day? 10-15GB of MP3 is a hell of allot of hours. If you put that on shuffle, you won't see many repeats in even a week of driving all day. Odds are there is allot of crap in your collection you really don't want to hear at all let alone that often so why put that into the car?
If you are also planning on movies... Understand that is is a crime to have most movies on a hard drive. Even if you own the original DVD. This is thanks to Digital Milenium Copy Right act and now other laws that are signed or pending. (Not sure what all has actually been signed right now.) The act of cracking the copy protection to get it on your HD is what makes it criminal. Considering that some police will look for even the smallest excuse to chuck you in jail, having movies on a HD isn't so good on the road even if it makes an insane amount of sense for other reasons. (Like you don't want to damage disks etc.)
The first chance they get they will be looking at the laptop for evidence of crimes. You can thank a number of sick freaks and criminal hackers for that.
Also understand that if the movies are visible to the driver while moving, they may be considered distracted driving and illegal.
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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurasic Park)
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