This is deep in the firmware, but happens continually in the background, all transparent to the user.

If the drive is still not ok it will map that sector to one of the spare sectors. There is a 3 times rule in the drive's firmware - it reads a sector 3 times and if all 3 times it is bad then it may do a 'recalibrate' on the fly, and read 3 more times. This is all transparent to the outside world - except for the SMART util.Įach manufacturer can do as they please, so some set the error counts to zero, even though there might be 10 bad sectors as soon as the drive is manufactured. In fact they are 'logically swapped out' - the bad sector is mapped to a new, good, spare cylinder sector (it has spare cylinders - think of cylinders as tracks). So they map them out, this means the drive skips bad sectors. In the olden days, we had to enter the bad sectors in to the HDD controllerįrom a list on paper that came in the new drive box, so the controller skips them.

There is no perfect HDD, never was, never will be, (history and fact). You will see how HDD's are made, tested and how they really work. If you read the data sheets (white papers) say at

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