Editing Hard disk drive (section)

== Integrity and failure ==
{{Main|Hard disk drive failure|Head crash|Data recovery}}
{{See also|Solid-state drive#SSD reliability and failure modes|RAID#Unrecoverable read errors during rebuild}}
{{More citations needed section|date=January 2025|talk=More citations needed in integrity section}}
[[File:Hard disk head.jpg|thumb|Close-up of an HDD head resting on a disk platter; its mirror reflection is visible on the platter surface. Unless the head is on a landing zone, the heads touching the platters while in operation can be catastrophic.]]
[[File:Hard disk head crash.jpg|thumb|HDD head crash]]
[[File:Broken HDD head (kaputt).jpg|thumb|Even worse crash]]

Due to the extremely close spacing between the heads and the disk surface, HDDs are vulnerable to being damaged by a [[head crash]]&nbsp;– a [[hard disk failure|failure of the disk]] in which the head scrapes across the platter surface, often grinding away the thin magnetic film and causing data loss. Head crashes can be caused by electronic failure, a sudden power failure, physical shock, contamination of the drive's internal enclosure, wear and tear, [[corrosion]], or poorly manufactured platters and heads.

The HDD's spindle system relies on [[air density]] inside the [[disk enclosure]] to support the heads at their proper [[flying height]] while the disk rotates. HDDs require a certain range of air densities to operate properly. The connection to the external environment and density occurs through a small hole in the enclosure (about 0.5&nbsp;mm in breadth), usually with a filter on the inside (the ''breather filter'').<ref name="AutoMK-80" /> If the air density is too low, then there is not enough lift for the flying head, so the head gets too close to the disk, and there is a risk of head crashes and data loss. Specially manufactured sealed and pressurized disks are needed for reliable high-altitude operation, above about {{convert|3000|m|ft|abbr=on}}.<ref name="AutoMK-81" /> Modern disks include temperature sensors and adjust their operation to the operating environment. Breather holes can be seen on most disk drives, excluding sealed drives, such as drives that use helium, where any exposure to outside air would cause a failure&nbsp;– they usually have a sticker next to them, warning the user not to cover the holes. The air inside the operating drive is constantly moving too, being swept in motion by friction with the spinning platters. This air passes through an internal recirculation filter to remove any leftover contaminants from manufacture, any particles or chemicals that may have somehow entered the enclosure, and any particles or outgassing generated internally in normal operation. Very high humidity present for extended periods of time can corrode the heads and platters. An exception to this are hermetically sealed, helium-filled HDDs that largely eliminate environmental issues that can arise due to humidity or atmospheric pressure changes. Such HDDs were introduced by HGST in their first successful high-volume implementation in 2013.

For [[giant magnetoresistive]] (GMR) heads in particular, a minor head crash from contamination (that does not remove the magnetic surface of the disk) still results in the head temporarily overheating, due to friction with the disk surface and can render the data unreadable for a short period until the head temperature stabilizes (so-called "thermal asperity", a problem which can partially be dealt with by proper electronic filtering of the read signal).

When the logic board of a hard disk fails, the drive can often be restored to functioning order and the data recovered by replacing the circuit board with one of an identical hard disk. In the case of read-write head faults, they can be replaced using specialized tools in a dust-free environment. If the disk platters are undamaged, they can be transferred into an identical enclosure and the data can be copied or cloned onto a new drive. In the event of disk-platter failures, disassembly and imaging of the disk platters may be required.<ref>{{cite web|url=http://electronics.howstuffworks.com/how-to-tech/how-to-recover-data-hard-drive4.htm|title=How To Recover Lost Data from Your Hard Drive|last=Grabianowski|first=Ed|publisher=HowStuffWorks|pages=5–6|access-date=October 24, 2012|date=May 29, 2009|archive-url=https://web.archive.org/web/20121105121608/http://electronics.howstuffworks.com/how-to-tech/how-to-recover-data-hard-drive4.htm|archive-date=November 5, 2012|url-status=live}}</ref> For logical damage to file systems, a variety of tools, including [[fsck]] on [[UNIX-like]] systems and [[CHKDSK]] on [[Windows]], can be used for [[data recovery]]. Recovery from logical damage can require [[file carving]].

A common expectation is that hard disk drives designed and marketed for server use will fail less frequently than consumer-grade drives usually used in desktop computers. However, two independent studies by [[Carnegie Mellon University]]<ref name="CMUDiskFailure">{{cite web |title=Everything You Know About Disks Is Wrong |url=https://storagemojo.com/2007/02/20/everything-you-know-about-disks-is-wrong/ |website=Storagemojo.com |date=February 22, 2007 |access-date=May 24, 2019 |archive-url=https://web.archive.org/web/20190524230125/https://storagemojo.com/2007/02/20/everything-you-know-about-disks-is-wrong/ |archive-date=May 24, 2019 |url-status=live }}</ref> and [[Google]]<ref name="GoogleDiskFailure">{{cite web |title=Failure Trends in a Large Disk Drive Population |first1=Eduardo |last1=Pinheiro |author2=Wolf-Dietrich Weber |author3=Luiz André Barroso |url=http://research.google.com/archive/disk_failures.pdf |publisher=Google Inc |date=February 2007 |access-date=December 26, 2011 |archive-url=https://web.archive.org/web/20100105013605/http://research.google.com/archive/disk_failures.pdf |archive-date=January 5, 2010 |url-status=live }}</ref> found that the "grade" of a drive does not relate to the drive's failure rate.

A 2011 summary of research, into SSD and magnetic disk failure patterns by [[Tom's Hardware]] summarized research findings as follows:<ref>[http://www.tomshardware.com/reviews/ssd-reliability-failure-rate,2923-9.html Investigation: Is Your SSD More Reliable Than A Hard Drive?] – [[Tom's Hardware]] long term SSD reliability review, 2011, "final words"</ref>
* [[Mean time between failures]] (MTBF) does not indicate reliability; the annualized failure rate is higher and usually more relevant. 
* HDDs do not tend to fail during early use, and temperature has only a minor effect; instead, failure rates steadily increase with age.
* S.M.A.R.T. warns of mechanical issues but not other issues affecting reliability, and is therefore not a reliable indicator of condition.<ref>{{cite web|last1=Anthony|first1=Sebastian|title=Using SMART to accurately predict when a hard drive is about to die|date=November 12, 2014 |url=http://www.extremetech.com/computing/194059-using-smart-to-accurately-predict-when-a-hard-drive-is-about-to-die|publisher=ExtremeTech|access-date=August 25, 2015|archive-url=https://web.archive.org/web/20150831050118/http://www.extremetech.com/computing/194059-using-smart-to-accurately-predict-when-a-hard-drive-is-about-to-die|archive-date=August 31, 2015|url-status=live}}</ref>
* Failure rates of drives sold as "enterprise" and "consumer" are "very much similar", although these drive types are customized for their different operating environments.<ref>{{cite web|title=Consumer hard drives as reliable as enterprise hardware|work=Alphr |date=December 4, 2013 |url=http://www.alphr.com/technology/23998/consumer-hard-drives-as-reliable-as-enterprise-hardware|access-date=August 25, 2015|archive-url=https://web.archive.org/web/20150911033552/http://www.alphr.com/technology/23998/consumer-hard-drives-as-reliable-as-enterprise-hardware|archive-date=September 11, 2015|url-status=live}}</ref><ref>{{cite web|last1=Beach|first1=Brian|title=Enterprise Drives: Fact or Fiction?|url=https://www.backblaze.com/blog/enterprise-drive-reliability/|publisher=Backblaze|access-date=August 25, 2015|date=December 4, 2013|archive-url=https://web.archive.org/web/20150818015807/https://www.backblaze.com/blog/enterprise-drive-reliability/|archive-date=August 18, 2015|url-status=live}}</ref>
* In drive arrays, one drive's failure significantly increases the short-term risk of a second drive failing.

{{As of|2019}}, Backblaze, a storage provider, reported an annualized failure rate of two percent per year for a storage farm with 110,000 off-the-shelf HDDs with the reliability varying widely between models and manufacturers.<ref name= "Backblaze 3Q2019" >{{cite web |title= Hard Drive Data and Stats |url= https://www.backblaze.com/b2/hard-drive-test-data.html |publisher=Backblaze |access-date= November 24, 2019 }}</ref> Backblaze subsequently reported that the failure rate for HDDs and SSD of equivalent age was similar.<ref name="BBRel2021">{{cite web
 |url=https://www.backblaze.com/blog/are-ssds-really-more-reliable-than-hard-drives/ 
 |title=Are SSDs Really More Reliable Than Hard Drives?  |last=Klein  |first=Andy
 |date=September 30, 2021  |website=Backblaze  |access-date=September 30, 2021
 |quote=Once we controlled for age and drive days, the two drive types were similar and the difference was certainly not enough by itself to justify the extra cost of purchasing a SSD versus a HDD.}}</ref>

To minimize cost and overcome failures of individual HDDs, storage systems providers rely on redundant HDD arrays. HDDs that fail are replaced on an ongoing basis.<ref name= "Backblaze 3Q2019"/><ref name= "HAMR 2008 for 2009"/>