Bad sector

Bad sector

A bad sector is a sector on a computer's disk drive that cannot be used due to permanent damage, such as physical damage to the disk particles. It is usually detected by a disk utility software such as CHKDSK or SCANDISK on Microsoft systems, or badblocks on Unix-like systems. When found, these programs mark the sectors unusable and the operating system skips them in the future.

A modern hard drive comes with many spare sectors. When a sector is found to be bad by the firmware of a disk controller, the disk controller remaps the logical sector to a different physical sector. In the normal operation of a hard drive, the detection and remapping of bad sectors should take place in a manner transparent to the rest of the system. When the operating system begins to detect bad sectors, in most cases, it means that the surface of the hard disk is failing and the drive has run out of spare sectors with which to remap the failed sector. There are a variety of utilities that can read the SMART information to tell how many sectors have been reallocated, and how many spare sectors the drive may still have. [ [http://www.linuxjournal.com/article/6983 Monitoring Hard Disks with SMART] . Linux Journal, 2004.]

A commonly held idea for operation with regard to bad sectors is that automatic remapping of sectors only happens when a sector is re-written to or re-written with zero-filled data. The logic behind this is presumably that even if a sector is having trouble reading it may still be readable with data recovery methods. However, if a drive knows that a sector is bad and the drive's controller receives a command to write over it it will not reuse that sector and instead remap it to one of its spare-sector regions. This may be the reason why hard disks continue to have sector errors (mostly disk controller timeouts) until all the bad sectors are remapped: typically with an entire-drive zeroing of sectors. See the SMART attribute number 197 ("Current Pending Sector Count") for more information. [Citation
editor-last = Stephens
editor-first = Curtis E
title = Information technology - AT Attachment 8 - ATA/ATAPI Command Set (ATA8-ACS), working draft revision 3f
publisher = ANSI INCITS
date = December 11, 2006
url = http://www.t13.org/Documents/UploadedDocuments/docs2006/D1699r3f-ATA8-ACS.pdf
pages = 198-213, 327-344]

References

External links

* [http://users.iafrica.com/c/cq/cquirke/baddata.htm What causes bad sectors?]


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