The only time I've ever had a hard drive crash, it was not a heat problem. It was only 3 months old, and it was a manufacturing problem, I'm pretty sure. They were 500 Mb (LOL!) Seagate drives, and dozens of them crashed, all over the building. We'd had good luck with Seagate before, but that batch was buggy as all get-out. (We weren't the only ones having trouble.) They did replace the drives, but it was a pain to be without one while the exchange was made.

I have had to replace drives since then, but the newer drives tell you when they're getting bad, so I've always been able to do it before anything catastrophic happens.

I realize that you have to take user reviews with a grain of salt, but I can't help noticing the pattern. It doesn't seem like a heat problem. One person reported that half the drives he bought for his office didn't work, right out of the box. Another person complained that his drive crashed after ten days, and when he called to complain, they told him, "Well, every hard drive fails eventually." But you don't see complaints like that with drives that are under 180 Gb. Just look at the reviews at sites like that allow customer reviews. They list the stars by the product, and you can just look down the list, and see the trend clearly.

Anyway, I think most external hard drives have built-in fans, so heat shouldn't be a problem. Though maybe I should make a point of getting one with a metal case, instead of plastic. Plastic is too darn insulating.