I had an IBM 75GXP 45GB running 24/7 w/ Seti going while idle and I wouldn't say it was hot. If you touched it, it was warm. I got a second one, but since running them on top of each other would create a pocket of hot air, I stuck them in 5.25 bays and cut the bay covers and stuck 2 60mm fans over them. The fans blow over both hard drives and only cost $5 for nice Sunon fans. If you have a single drive, I don't think you need any extra cooling (depending how the ambient temperature is in your case), but I would not suggest putting any other 3.5 drives next to it.
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