A bad sector is a sector on a computer's disk drive or flash memory that is either inaccessible or unwriteable due to permanent damage, such as physical damage to the disk surface or failed flash memory transistors. Bad sectors are 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 may mark the sectors unusable (most file systems contain provisions for bad-sector marks) and the operating system skips them in the future.