HDD Manufacturing Defect Mapping
Defect mapping is performed during the manufacture of the HDD. A Surface Analysis Test (SAT) is performed whereas the disk is written repeatedly with stressed data patterns and subsequently read back. Any sectors that cannot be read back successfully are removed from the sector map. A list of bad sectors is kept within the drive (called the P-list). After the SAT, the drives are subjected to a final test which may uncover additional media defects. Those sectors are also mapped out and the P list adjusted. Additional testing is performed during System Manufacturing to detect any defects which may have not been mapped out by the HDD manufacturer (unlikely) or which may have 'grown' due to latent media defects. (possible).
Defect discovery at System Manufacturing is rare but is nevertheless performed to ensure that the drives are defect free when shipped to our customers.
As drives are used in the field, grown defects can continue to occur due to many circumstances: latent imperfection on the disk, media damage due to mishandling of the drive and harsh environments.
However, these defects can be reallocated by the drive to available spare sectors in the drive.