Although the fabrication of hard disk platters has improved immeasurably, but unreadable bad sectors will always be an issue during the lifetime of the drive. Unreadable bad sectors can cause a lot of problems, making it essential to keep track of any which exist at the time of manufacture and any which develop during its lifetime.
During data recovery it is important to know the precise location of each bad sector, particularly those which existed at the time the drive was factory formatted. This is achieved by keeping track of bad sectors maintaining two list with allow the bad sectors to be mapped out during operation.
Primary Defect List (P-List) Factory Format
In spite of the latest improvements in platter fabrication there is always the chance of bad sectors being found during the factory format process. These are stored in a primary list (P-List) which is used to shift the offset required to read all subsequent sectors on the disk. Should the P-List become corrupt or inaccessible, it will lead to incorrect sectors being returned. Such a failure, although rare, is usually the result of bad or firmware failure, often due to excessive heat.
Each bad sector found results in the sectors required being shifted for each one encountered. For example, if the 100th sector is damaged, physical sector 101 will become logical sector 100. Each sector is shifted by the number of sectors held in the P-List with an equal or lower sector number.
Grown Defect List (G-List)
When a hard disk is operating normally, each new bad sector encountered is mapped out to another in the spare sector area, storing the sector details into the grown defect list (G-List). The procedure is usually performed automatically by the drive firmware, usually without any data being lost in the process.
The spare sector area allocated is a fixed size, which means that once this area has been exhausted, any new bad sector will be returned to the operating system. This information will be stored in the S.M.A.R.T. data, but the operating system often does not report the presence of bad sectors unless they affect important file system data structures.
Implications for Data Recovery
Although it is rare for either of these lists to be damaged, corrupted or lost, when it occurs the consequences can cause severe issues. If the damage or corruption occurs to the P-List it will result the sectors not being correctly shifted for each bad sector which was detected during the factory format process. This will result in the underlying file system metadata structures not being the correct positions, a serious issue for most modern file systems.
If the grown defect list become corrupt, it will result in data loss, but only those sectors which had been swapped out or even read old data from the original sector. The hard disk drive manufacturers take many precautions to avoid this occurring. There have been several instances where a range of drives have suffered such an issue, leading to problems for the customer and manufacturer, which can severely damage their reputation.