If you've been using computers for a decent amount of time there is a good chance someone has told you that data stored on a hard drive is not exactly safe. I'm here to assure you that this is indeed true.
Never mind the fact that unlike tapes or CDs or other methods of storage, hard drives are mechanical, active devices and are thus subject to comparatively rapid breakdown.
No, the real threat to hard drives are the people that use them, by which I mean you and me. Hard drives, being the dynamic storage devices that they are, are extremely easy to erase in any number of amusing and simple to achieve ways...