Bad sector

A portion of a disk that cannot be used because it is flawed. When you format a disk, the operating system identifies any bad sectors on the disk and marks them so they will not be used. If a sector that already contains data becomes damaged, you will need special software to recover the data.Almost all hard disks have sectors that are damaged during the manufacturing process, but these are usually replaced with spare sectors at the factory. By the time the disk is shipped, it should be free of bad sectors. If your disk utility starts showing bad sectors, therefore, this is a sign that there is something wrong with the disk or disk drive.