Well, I don't have any experience with portable OS', if that even exists.
IMO, you'll still need HDD, unless you're willing to spend more for an SSD. For the reason you don't want to use HDD since it doesn't like bumpy ride in a storm, you can use a shock absorbing pad, bushing or any means similar to minimize the shock received. Which puts me in the same opinion as Wolygon.
Right away when I read your post I thought of SSDs. They don't have the same moving parts as normal HHD's, they don't require much electricity to run and are pretty sturdy over all (assuming your not smacking them with a hammer or something.)
It runs off of a USB Mini Port and is a PCI express. SO you buy a PCI-E USB Mini Card to put into a PCI-e slot on your PC like one found here; fficial&channel=s&hl=en&q=PC+express+card+expansion&um=1&ie=UTF-8&cid=6134401896167852382&ei=5yDOTPC1L8GBlAeB_-HjCA&sa=X&oi=product_catalog_result&ct=result&resnum=4&ved=0CC0Q8wIwAw#" rel="nofollow" target="_blank">http://www.google.com/products/catalog?client=firefox-a...
Then simply plug in the hard drive, it will run with very low head emissions, no moving parts, low power consumption and will work well in temperature fluctuations.
Total cost? Well you can get the harddrive for anywhere between $50 and $250 depending on the size and the cards go for $10 to $50 depending on how fancy you want to go. Don't forget you almost always get what you pay for, and it seems it would be in your best interest to spend what you can afford for this setup.