A magnetic disk sector that has become defective and cannot store data reliably. The usual cause is damage to the magnetic medium, and it is quite usual for disks to acquire bad sectors over time in normal usage. Such sectors can be marked as “bad”, traditionally by utility programs or the operating system, so that they are not used again. If possible their data is recovered and transferred to vacant good sectors. Modern disks initially contain unused sectors that can be transparently substituted for bad sectors by the disk controller, with the rest of the system being unaware that there has been a problem. The accumulation of many bad sectors is often a sign that the magnetic surface is starting to fail and that the disk should be replaced.