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Error Notification and Defect Mapping
Many drives are smart enough to realize that if a sector can only be read after retries, the chances are good that something bad may be happening to that sector, and the next time it is read it might not be recoverable. For this reason, the drive will usually do something when it has to use retries to read a sector (but usually not when ECC will correct the problem on the fly). What the drive does depends on how it is designed.
Modern drives support SMART, a reliability feature that tries to predict drive failure based on technological "leading indicators". Read errors, excessive numbers of retries, or problems seeking are very commonly included in the set of parameters used to signal impending hard drive doom. Activity of this sort that exceeds a safety threshold determined by the drive's designers may trigger a SMART warning, telling the user that the drive may be failing.
Today's hard disks will also often take corrective action on their own if they detect that errors are occurring. The occasional difficulty reading a sector would typically be ignored as a random occurrence, but if multiple retries or other advanced error correction procedures were needed to read a sector, many drives would automatically mark the sector bad and relocate its contents to one of the drive's spare sectors. In doing so, the drive would avoid the possibility of whatever problem caused the trouble worsening, and thereby not allow the data to be read at all on the next attempt.