Hard drives are mechanical devices, as such they wear out and are susceptible to failure. The most likely reason for a drive to fail prematurely is physical damage to the drive. If a drive is bumped or jostled while it is processing there is a chance that the process has failed. There are many other reasons why data on the hard drive may become corrupted or unusable. Hardware or other component failures (especially memory), poorly written software, or viruses in the system can all lead to erroneous data.
It is important to understand that a hard drive is made up of one or more spinning platters that contain tracks of magnetic information. The platters are read by the read/write head which sits on the surface of the disk on a cushion of air created by the speed of rotation of the platter. The read/write heads are attached to an armature driven by a magnetic coil that can move it over the surface of the disk.
The platters typically spin at speeds of 5400 to 7200 RPM, with read/write motion directed to separate areas of the drive almost instantaneously. When the hard drive is performing some process, the read/write heads are working on reading and/or writing data on the surface of the platters. At this point the heads come into contact with the platter surface, if the process is interrupted it can cause damage to both the heads and the platter surface.
Fortunately, modern units are well protected against this type of damage, especially if the heads are moving. All hard drives made in the last decade are protected when the drive heads are spinning. External hard drives or those included in laptops are better protected than desktop hard drives, however they can still be affected by a physical drop.
The electric motor that drives the rotation of the tray is also subject to failure after a long period of use. A drive motor failure can cause the drive head to take longer to read or write data.
Hard drives contain air filters that provide the internal atmosphere necessary to keep the read/write heads running across the surface of the drive. A faulty filtering system can allow particles to enter the drive mechanism, which can cause extensive data damage.
Any mechanical or physical failure of the drive will surely cause software problems (usually due to bad sectors) That’s why data should always be backed up. Of all the reasons, however, the actual mechanical failure of the hard drive is the most inevitable and catastrophic.