Take some advice from someone who has gone through four of these damn things. If you're working in an environment where they will be exposed to anything above about 80 deg, and you plan on leaving it on 24/7, get one with a built in fan (or two). All mine that died were passive heat dissipators (i.e., aluminum case), and they just couldn't handle higher temp conditions I exposed them to in a constant on state.
I still have one, a Seagate, that's working fine, but I only turn it on for maybe a few hours a day. The ones that died were all Maxtors (and I generally like Maxtor stuff).
You can find fan cooled externals online, or better yet, buy the parts and put it together yourself. Cheap and easy.
Edit: okay, just actually read ALL of your post and see you won't be using it constantly. For price though, building your own via extremepcgear.com or similar will still be the cheapest (plus you get the fan)
