Maintaining a spare sector per track to be used if another sector becomes defective. After detecting a bad write, the disk driver or controller writes the data into the spare sector and marks the bad sector as unusable. Existing data can also be moved to the spare sector if the sector is marginal, but the data can be read after several attempts. Sector sparing provides a degree of fault tolerance within the individual disk itself, but RAID provides fault tolerance for the entire disk. See RAID and hot fix.