I believe the multiple sets of files thing is what overheated my HD in the first place! I've decided to take it easy with my USB drives, and I'll never buy one without a nice, powerful fan again. If you don't need 750, you can just sell 'em off. I wouldn't mind another 250 gb. lol.
I got my enclosure at Wanchai Computer Centre. There's a store on the first floor that's more like a counter. When you go up the escalator, make a left and go straight. The counter will be on your left after you pass the first few stores. It's a Lapara brand enclosure. I wish I'd bought another one at the time!