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. A bad sector is typically formed as a result of physical damage of some sort, or rarely, the operating system's 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 disk utility software - such as SCANDISK or CHKDSK on Microsoft systems, or badblocks on Unix and Unix-like systems - it marks the sectors that have failed so that the OS can skip them in the future. All file systems contain specifications for bad sector marks.
Several spare sectors are located within modern hard drives. The firmware of a disk controller identifies bad sectors and then 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 never gets re-used. Instead, the drive controller just removes the sector address from the list of usable storage locations in ROM.Hard disks nearly all have bad sectors idetified in the manufacturing processed, the addresses of these bad sectors is held in the disk controller ROM so that these areas will not be used during any disk operations.When bad sectors appear after the manufacturing process, there is usually either an impact that causes the disk head to "crash" onto the rotating surface causing damage to the very delicate disk surface. It is similar to dropping the needle of a gramaphone record and scratching the vinyl. Deterioration of the recording surface can also cause bad sectors to appear. A drive that is showing bad sectors, especially if more are appearing regularly, should be backed up and replaced before severe data loss occurs. The bad sector list provided by the factory is called the P-List and bad sectors found after installation by the end user is called the G-List
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