Defects may also be detected and managed during execution of the FORMAT UNIT command. The FORMAT UNIT command defines four sources of defect information. These defects may be reassigned or avoided during the initialization process so that they do not appear in a logical block.
Defects may also be avoided after initialization. The initiator issues a REASSIGN BLOCKS command to request that the specified logical block address be reassigned to a different part of the medium. This operation can be repeated if a new defect appears at a later time. The total number of defects that may be handled in this manner can be specified in the mode parameters.
Defect management on direct-access devices is usually vendor-specific. Devices not using a removable medium typically optimize the defect management for capacity or performance or both. Devices that use a removable medium typically do not support defect management (e.g. some floppy disk drives) or use defect management that is based on the ability to interchange the medium.