The defects table in a hard disk drive consists of two lists: the P List and the G List. They contain records of all the defective (or ‘bad’) sectors in the hard disk & provide in-built data error prevention measures.
Bad sectors fall into two categories; those created during the manufacturing process and those which develop when the drive is in operation. Hard drives are designed to identify and contain bad sectors by the use of two defect tables.
Bad sectors encountered during post-manufacture testing are contained within a defect table known as the P-list (“permanent” or “primary” defect table). P-list sectors have no effect on drive access speed as they are automatically omitted by the drive electronics.
The G-list or “growth” defect table contains sectors which have become corrupted while the drive is in use. Although data operations are automatically redirected to uncorrupted sectors, the G-list table does reduce drive access speed and it may become necessary to replace the drive.
There are a number of hard drives that have apparent firmware issues. Damage is sustained to either the firmware code itself, or to some of the hard drive parameters located on the hard disk platter surface that are accessed by the firmware.