A hard disk drive is composed of one or more platters, spinning disks containing tracks of magnetic information. These platters are read by read/write heads that hover just above the surface of the disk on a cushion of air created by the speed of the platter's rotation. These heads are attached to an armature that can move them over the surface of the disk, powered by a magnetic 'voice coil'.

The whole assembly moves incredibly quickly and precisely. The platters generally rotate at either 5400 or 7200 RPM, and the movement of the read/write heads to separate areas of the disk is almost instantaneous.

Hard disks will fail eventually because they are mechanical devices, and as such, wear out. It's as simple as that. If a drive is bumped or jostled while it is active (meaning that the read-write heads are busy reading or writing data on the surface of the platters) there is a chance that the heads will make contact with the surface of the platter, which can cause all sorts of problems.

This 'head-crash' can cause damage both to the read heads and the surface of the platter, can knock the read-heads out of proper alignment, and a host of other badness.

Fortunately, modern drives are well protected against this kind of damage while the heads are in motion. All hard drives manufactured in the last decade or so protect themselves automatically when the drive spins down by parking the read-write heads, or lowering them onto a safe area on the surface of the stopped platter.

A stopped drive is extremely difficult to damage by physical means. For this reason, you should avoid moving your computer around while it is powered on. Obviously, most laptop hard disks are better protected than standard 3-1⁄2 inch desktop hard drives, but they can still be affected by physical force.

The electric motor that powers the platter's rotation is also subject to failure over a long period of use. A failure in the drive's motor or bearings can cause slow performance or data read/write errors due to the platters spinning up slowly or rotating at an incorrect speed.

Hard drives contain filtered air-intakes, to provide the internal atmosphere needed to keep the read/write heads hovering above the surface of the disk. A failure in the filtering can allow particles into the drive mechanism, which can quickly cause great damage to your data.
Any mechanical failure of the disk, or physical damage inflicted to it will almost certainly cause attendant software problems, generally due to bad sectors.

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