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hard disk drive failure
A hard disk drive failure occurs when a hard disk drive malfunctions and the stored information cannot be accessed with a properly configured computer. A disk failure may occur in the course of normal operation, or due to an external factor such as exposure to fire or water or high magnetic fields, or suffering a sharp impact or environmental contamination, which can lead to a head crash.
== Causes ==
Some hard drives simply fail because of worn out parts, others fail prematurely. HDD manufacturers typically specify a Mean Time Between Failures or an Annualized Failure Rate (AFR) which are population statistics that can not predict the behavior of an individual unit. 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. HDD failures tend to follow the concept of the bathtub curve.〔(【引用サイトリンク】url=http://www.extremetech.com/computing/170748-how-long-do-hard-drives-actually-live-for )〕 HDDs typically fail within a short time if there is a defect present from manufacturing. If an HDD proves reliable for a period of a few months after installation, the HDD has a significantly greater chance of remaining reliable. Therefore, even if a HDD 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, an HDD can fail at any time in many different situations.
The most notorious cause of HDD 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. HDD 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 HDDs equalize the atmospheric pressure and moisture between the HDD 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 an HDD crash, particles from the damaged platter and head media can cause one or more bad sectors. These, in addition to platter damage, will quickly render a HDD useless. An HDD also includes controller electronics, which occasionally fail. In such cases, it may be possible to recover all data.
The phenomenon of disk failure is not limited only to HDDs, but also applies to other types of magnetic media. In the late 1990s, Iomega's 100-megabyte Zip disks used in Zip drives were affected by the click of death, called so because the drives endlessly clicked when accessed, indicating the impending failure. 3.5-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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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