During a low-level format, defect lists are populated, which store a list of bad sectors, which are then mapped and a sector slipping algorithm is utilized. When using sector slipping for bad sectors, disk access time is not largely affected. The drive will skip over a bad sector using the time it would have used to read it. Spare sectors are located on the disk to aid in having sectors to “slip” other sectors down to, allowing for the preservation of sequential ordering of the data. Accuracy of programs, reliant on static knowledge of cylinders and block positions will be compromised, however.
Bad sectors that are found during normal usage of the disk are not capable of having the sector slipping algorithm applied. Instead, a linear reallocation, or sector forwarding, is used where a bad sector is replaced with a sector from a spare area. Doing this affects the access times, as the disk will need to seek to the spare sector since all further lookups of the bad sector will redirect to the new sector.