Bad sector
Also known as: bad block, disk error, damaged sector, bad sectors
A bad sector is a small region of a hard drive or storage device that can no longer reliably store data, due to physical damage or wear. The drive marks bad sectors so it stops using them; a growing number of them is a warning that the drive is failing.
- A region of a drive that can no longer store data reliably
- Hard sectors are physical damage; soft ones are repairable errors
- A rising count is a sign the drive is failing
Physical vs logical bad sectors
A physical (hard) bad sector is genuinely damaged — from manufacturing defects, wear, or physical shock — and cannot be repaired; the data it held may be lost. A logical (soft) bad sector is intact but contains an error, and can often be reset by a disk repair tool that rewrites it.
On a traditional hard disk drive, sectors are physical spots on a spinning platter. Drives keep a small reserve and remap bad sectors to spare ones automatically, hiding the problem until the spares run out.
Why a rising count matters
A few remapped sectors are normal over a drive's life, but a steadily climbing count signals the drive is wearing out and should be backed up and replaced. Disk tools and SMART health data report reallocated and pending sectors.
Solid-state drives have no sectors in the mechanical sense; an SSD manages worn flash cells through its own wear-leveling and spare blocks, so "bad sector" most precisely describes spinning hard drives.