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A bad sector is an unusable part or subdivision within a track on a magnetic or optical disc located on a computer's hard disk or flash drive. It typically forms as a result of physical damage or, rarely, the operating system's (OS) inability to access the information.
The physical damage occurs to the disk surface or as a result of flash memory transistor failure. Once the bad sector is identified by the disk utility software – such as SCANDISK or CHKDSK on Microsoft systems, or badblocks on Unix and Unix-like systems – it marks the failed sectors so that the OS can skip them in the future. All file systems contain specifications for bad sector marks.
A bad sector is also known as a bad block.
Several spare sectors are located in modern hard drives. The firmware of a disk controller identifies bad sectors and remaps them to a different physical sector. When a bad sector is pinpointed, automatic remapping occurs; this automated process typically takes place when a sector is overwritten. The sector is never reused. Instead, the drive controller just removes the sector address from the list of usable storage locations in ROM.
Nearly all hard disks have bad sectors identified during the manufacturing process, and the addresses of these bad sectors are held in the disk controller ROM, allowing these areas to remain used during any disk operation. When bad sectors appear after the manufacturing process, there is usually an impact that causes the disk head to crash on the rotating surface causing damage to the very delicate disk surface. It is similar to dropping the needle of a gramophone record and scratching the vinyl.
Deterioration of the recording surface also can cause the appearance of bad sectors. A drive that shows bad sectors, especially if more appear on a regular basis, should be backed up and replaced to prevent severe data loss.
The bad sector list provided by the factory is known as the P-List, and bad sectors found after end user installation comprise what is known as the G-List