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Since the read/write heads of a hard disk are floating on a microscopic layer of air above the disk platters themselves, it is possible that the heads can make contact with the media on the hard disk under certain circumstances. Normally, the heads only contact the surface when the drive is either starting up or stopping. Considering that a modern hard disk is turning over 100 times a second, this is not a good thing. :^)
If the heads contact the surface of the disk while it is at operational speed, the result can be loss of data, damage to the heads, damage to the surface of the disk, or all three. This is usually called a head crash, two of the most frightening words to any computer user. :^) The most common causes of head crashes are contamination getting stuck in the thin gap between the head and the disk, and shock applied to the hard disk while it is in operation.
Despite the lower floating height of modern hard disks, they are in many ways less susceptible to head crashes than older devices. The reason is the superior design of hard disk enclosures to eliminate contamination, more rigid internal structures and special mounting techniques designed to eliminate vibration and shock. The platters themselves usually have a protective layer on their surface that can tolerate a certain amount of abuse before it becomes a problem. Taking precautions to avoid head crashes, especially not abusing the drive physically, is obviously still common sense. Be especially careful with portable computers; I try to never move the unit while the hard disk is active.