PortalSoup, welcome to the forum. I would think that any drive manufacturer would recommend against using a belt clip for a hard drive. Bumping, jarring, or dropping a hard drive is bad and can result in failure. I can't tell you how many times I have bumped, jarred, and dropped my cell phone from it's holster on my belt.
But then, as if to contradict myself, I have carried a hard drive clipped to my belt. My music player, a Creative Zen, has a 30 GB hard drive inside and I have a belt clip for it. I'm far more careful with this device and it is in a padded body glove. I don't believe that I've ever dropped it. So, if your portable hard drive is about the size of an iPod or other music player, you could probably find a belt clip. If it's much bigger than that, it's probably to clunky to hang off your belt.