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.
A modern hard disk is turning over 100 times a second. 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.
When the platters are not spinning, the heads rest on the surface of the disk. When the platters spin up, the heads rub along the surface of the platters until sufficient speed is gained for them to lift off and float on their cushion of air. When the drive is spun down, the process is repeated in reverse. In both of the cases, for a period of time the heads make contact with the surface of the disk while in motion.
While the platters and heads are designed with the knowledge in mind that this contact will occur, it still makes sense to avoid having this happen over an area of disk where there is data.
For this reason, most disks set aside a special track that is designated to be where the heads will be placed for takeoffs and landings. This area is called the landing zone, and no data is placed there. The process of moving the heads to this designated area is called head parking.
Almost all new operating systems have inbuilt facility to park the head automatically when it is necessary. Most early hard drives that used stepper motors did not automatically park the heads of the drive therefore as a safety precaution many small utilities were written that the user would run before shutting down the PC of those days. The utility would instruct the disk to move the heads to the landing zone, and then the PC could be shut off safely.
A parameter in the BIOS setup for the hard disk tells the system which track was the landing zone for the particular model of hard disk. Usually, it was the next consecutive-numbered track above the largest-numbered one actually used for data. Modern voice-coil actuated hard disk drives are all auto-parking. It is not necessary now to manually park the heads of modern hard disks.