Bad sector
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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 remaping 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.[citation needed]
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