No matter how meticulous the preparation and manufacturing of the disk platters and other components in a hard disk drive, the issue of unreadable bad sectors will always be a problem which must be addressed at some point during the lifetime of the drive. Each unreadable bad sector may result in the loss of data or file system metadata structures being damaged, which is the reason it is of extreme importance to correct the data and keep track of them during the lifetime of the drive.
During the data recovery process, it is important that the drive knows the precise location of each unreadable bad sector, particularly any which were encountered during the factory format procedure. A pair of lists is maintained detailing the location of all the bad sectors encountered, both during the factory format and those which develop during the operational life of the drive.
Primary Defect List (P-List)
While the quality of the platter fabrication process has improved immensely, there has also been a huge increase in the areal recording density, which means there remains a chance that some unreadable bad sectors may be found during the factory format process. The location of any unreadable bad sector detected during the factory format is stored in the primary defect list (P-List) which is used to instruct the firmware which sector should be skipped. For each of these sectors, the physical address of subsequent sectors is shifted by one for each bad sector located. As an example, it means that If physical sector 100 is damaged, that sector is stored in the P-List and physical sector 101 becomes logical sector 100.
In the unlikely event of damage to the P-List occurring during the operational lifetime of the drive, the wrong sectors will be returned, leading to corruption and failure of the file system. This condition usually only occurs rarely, often because of the drive running for long periods of time at a high temperature or due to damage caused by a power surge.
Grown Defect List Generation (G-List)
Hard disk drive will at some point, through general wear and tear or other factors, such as heat, impact damage, etc. slowly deteriorate until bad sectors become apparent. An area of the drive is allocated during the factory format, known as the spare sector area, which is used to relocate the data from a bad sector, at the same time generation a remapping entry in the grown defect list (G-List) which allows the drive firmware to automatically overcome bad sectors.
These bad sectors will however continue to develop, which will eventually fill the spare sector area. Once the spare sector area has been filled up, any new unreadable bad sectors which develop can no longer be mapped out, at which point the drive will report it to the operating system. Although the drive’s S.M.A.R.T. data contains information detailing a lot of information about the drive, such as the error rates and number of remapped sectors, this data on some occasions not be used by the operating system.
Data Recovery with Bad Sectors
It is fortunately, rare for the two bad sector lists to become corrupted or lost, as these are essential to ensuring the correct sectors are returned to the operating system. All the G-List is grown during the lifetime of the drive, the P-List is of much greater importance. If either or both lists become corrupted, it is important not to panic, but call DiskEng for immediate assistance.
The G-List is still important, is any damage may result in the drive ignoring the swapped our sector and make attempts to read the original one. This not only adds extra wear and tear during the recovery process, but may also lead to some data loss and corruption of the file system. Although it is also rare, there have been some high-profile incidents where a range of drives has suffered such a issue. This not only causes a lot of inconvenience for the customer, but also damages the drive manufacturer’s reputation.