Causes
The most notorious cause of hard-disk failure is a head crash, where the internal read-and-write head of the device, usually just hovering above the surface, touches a platter, or scratches the magnetic data-storage surface. A head crash usually incurs severe data loss, and data recovery attempts may cause further damage if not done by a specialist with proper equipment. Hard-drive platters are coated with an extremely thin layer of non-electrostatic lubricant, so that the read-and-write head will simply glance off the surface of the platter should a collision occur. However, this head hovers mere nanometers from the platter's surface which makes a collision an acknowledged risk. Another cause of failure is a faulty air filter. The air filters on today's hard drives equalize the atmospheric pressure and moisture between the hard-drive enclosure and its outside environment. If the filter fails to capture a dust particle, the particle can land on the platter, causing a head crash if the head happens to sweep over it. After a hard-drive crash, each particle from the damaged platter and head media can cause a bad sector. These, in addition to platter damage, will quickly render a hard drive useless. A hard drive also includes controller electronics, which occasionally fail. In such cases, it may be possible to recover all data.
Since hard drives are mechanical devices, they will all eventually fail. While some may not fail prematurely, many hard drives simply fail because of worn out parts. Many hard-drive manufacturers include a Mean Time Between Failures figure on product packaging or in promotional literature. These are calculated by constantly running samples of the drive for a short amount of time, analyzing the resultant wear and tear upon the physical components of the drive, and extrapolating to provide a reasonable estimate of its lifespan. Since this fails to account for phenomena such as the aforementioned head crash, external trauma (dropping or collision), power surges, and so forth, the Mean Time Between Failures number is not generally regarded as an accurate estimate of a drive's lifespan. Hard-drive failures tend to follow the concept of the bathtub curve. Hard drives typically fail within a short time if there is a defect present from manufacturing. If a hard drive proves reliable for a period of a few months after installation, the hard drive has a significantly greater chance of remaining reliable. Therefore, even if a hard drive is subjected to several years of heavy daily use, it may not show any notable signs of wear unless closely inspected. On the other hand, a hard drive can fail at any time in many different situations.
The phenomenon of disk failure is not limited to hard drives. Other media types are prone to failure; in the late 1990s the click of death, so called because affected drives would endlessly click when disks were inserted into them, plagued many users of Iomega's 100 megabyte Zip disks.
CD-ROM and DVD writeable media can fail over time due to degradation of the organic dye layer. Studies done by NIST under harsh conditions of light, temperature and humidity demonstrated sharp increases in bit errors after only 100 hours (with the exception of gold/phthalocyanine technology, which is far more durable). Advertised to last 100–300 years, the NIST report suggests that gold-layer disks are at least stable for "several tens of years", when stored properly. Drives with ever-increasing read and write speeds rotate CD and DVD media at over 25,000 rpm. Disks have been demonstrated to crack at 30,000 rpm due to centrifugal force.
3½-inch floppy disks can also fall victim to disk failure. If either the drive or the media is dirty, users may experience the buzz of death when attempting to access the drive.
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