netmasta got most of it. Hard drives are very touchy beasts and it's actually quite amazing that they last for as long as they do, considering the mechanics behind them. Consider that a lot of newer hard drives are spinning at 7200RPM. If your car ever went that fast, your engine would likely catch on fire. Aside from that, even though hard drives are very carefully sealed to be air tight, the heads that hover over the top of the platters inside your hard drive to read them are actually sitting so close above those platters that a single spec of dust that got in before the hard drive was sealed could be 2 or 3 times the distance between the platter and the head in diameter. To get a full picture of just what this would do if your head happened to hit a piece of dust,...
picture what a 747 would look like if its wheels touched down on a 50 gallon metal trashcan that was lying in the middle of the runway.
Yeah, can you hear me now?