For the past few days I've had an external drive resting comfortably on top of my subwoofer. It's an ideal location. Since the subwoofer is rather high and nearby, it's easy to see and monitor. Since the subwoofer seems cooler than the floor (and is wood as opposed to plastic) it should lower the temperature a bit as well. I haven't been playing a lot of music, but when I was blasting a track on high volume today I realized that maybe the location has some side-effects as well. Vibrations aren't good for any hard drives, though they seem pretty robust the mechanism that reads the disc is pretty fragile. Insanely tough compared to old hard drives (they'd probably have broken already after being placed on the sub) but fragile nevertheless. So I did some research.

1) The subwoofer is not a good location for the drive, partially because of vibrations but also because of the strong magnetism within such a device. Especially older subwoofers have very high magnetism. Older hard drives have much less resistance against both vibration and magnetism, new ones can probably handle the vibrations, as long as you don't max the volume/effect on a strong sub, but not the magnetism.

2) Sound itself has a harmful effect on hard drives. Regular conversations and music on a normal level probably won't do any (noticeable) damage, but you shouldn't constantly scream too near the drive. It reduces performance. This and the above is applicable on long-term usage.

3) Subwoofers for computers are built with a case that's supposed to shield other devices (like hard drives) from magnetism. As long as you leave a little space between the subwoofer and for example your computer, everything should be OK.

4) Hard drives without a case, such as those you put in a docking station to reach files on, should really have a case.

Also, I should probably get some antistat bags to store old drives in. Did you learn something? Hope you did, I know I did. In the future I think any external drives will be better off on the floor. :)