Hard Drive Bad Sectors

Hard Drive Bad Sectors

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Hard Drive Bad Sectors.

Defective areas of  digital storage media termed bad sectors affect the performance and operation of both hard disk drives (HDD’s  and solid state hard drives (SSD’s).

Hard Disk Drive Bad Sectors.

The recording media of a single hard drive disk will have over a billion sectors formatted on its surface. These sectors contain structured magnetic patterns that represent data. In production a number of these sectors will inherently be of poor magnetic recording quality and as a result be unusable or referred to as “bad sectors.”  These bad sector locations are mapped in a permanent defect list (P-List) that is stored in what is referred to as the firmware modules of the hard disk drive device. These firmware modules are a part of the data read and write processes of the device and the P-List ensures only good areas of the recording media that are produced in manufacture are used.

During normal operation a hard drive disk platter will develop further bad sector locations, these areas are mapped in what is called a grown defects list (G-List) in the device firmware. Bad sector areas are mapped and avoided and good sectors allocated for use. The OS uses the hard disk drive P –List sand G- List to translate the expected logical block address (LBA ) to a revised and valid read write address location. Such that information is not lost once a bad sector read operation is detected, a reserved or spare sector pool area of the recording media is made available to replace these grown bad sectors.

Solid State Hard Drive Bad Sectors.

Flash NAND SSD’s use metal–oxide–semiconductor field-effect transistors that store data logic states. Hard Drive SSD chips contain millions of integrated transistors on each device, providing the basic switching functions required to implement data storage. Cell and chip packing densities required to fit in modern portable storage devices imposes stringent quality tolerances both in manufacture, assembly and use.  SSD ‘s have an embedded  service area or controller for reading and writing to the individual cells and managing and distributing data on the drive in a process called wear levelling that improves performance and longevity of the drive. Cells are grouped into pages and pages are grouped into blocks, typically 128 pages comprise a 512KB block. Similar to a hard disk sectors, groups of SSD cells can become damaged and will be marked as a bad block (sector) by the service area controller.

Hard Drive S.M.A.R.T.

Self monitoring, analysis and reporting technology (SMART) is an underlying process that runs within a hard drive and maintains error logs and performance indicators that can be accessed by operating systems such as Windows or by special monitoring applications.  When an acceptable threshold of detected bad sectors is exceeding error count logs are produced that act as a warning to the system user.

Bad Sectors Overview.

In summary:

  • The P-list contains defects found during the final stages of the manufacturing process.
  • The G-List contains defects that develop during use of the drive

When your system is booted up the P-list and G-list are loaded into RAM as part of the initialisation process of the hard drive system module and this acts as a LBA translation pointer.

What Causes Hard Drive Bad Sectors.

Increasing numbers of bad sectors are caused by manner factors, stresses (wear ),  environmental conditions and misuse are common. Older hard drives subject to normal use generally have more tolerable bad sectors than “out of the box”. Hard drives with higher utilization will also generally exhibit more tolerable bad sectors than a hard drive with normal use.

Hard Drive Bad Sector Remapping.

All modern hard disk drives have a spare sector pool i.e. a reserved area of the recording media that can be indexed in place of sector that is marked as a bad sector . This is used when bad sectors develop during the normal life of the hard disk and any new bad sectors are ‘replaced’ with good ones from the spare sector pool.

P-list remapping

When bad sectors are found during manufacture  shift-points are entered into the P-List. eg. for an LBA sequence 0,1,2,3,4,5,6,7,8,9,10 …99  with  bad sectors 3, 6 and 9. The index of available sectors is translated as  0,1, 2, P, 3, 4, P, 5, 6, 7,P, 8, 9,10 .  with a bad sector count of three. The bad sectors are mapped out of the Logical Block Address and  replaced with addresses in the spare sector pool.  System  performance is not noticeably affected.

G-List remapping

The G-List contains the location of bad sector defects found during normal operation.  eg. for a bad sector at  LBA 6. What happens in this case is first the bad sector is mapped out. Giving; 0,1,2,3,4,5,G,7,8,9,10 and a good sector from the spare pool is indexed in the bad sectors place.  The LBA is an absolute index value to all the data stored on a hard drives recording media.

Once the spare sector pool is exhausted, the hard drive SMART technology will  show a critical state and a data back-up is needed.

It is possible to recover data directly from sectors recorded in the G-list. and this accounts for a change in  government regulations that mandate that sensitive data stored on hard drives must be physically destroyed once they have failed or when computers are upgraded.

Hard Drive Bad Sector Data Recovery.

Hard drives that have an excessive number of bad sectors will normally exhibit slow OS response times or produce error reports under normal operation. Data stored in newly detected bad-sectors will be lost and show as files that no longer open or as applications that fail to run.  Storage media “bad sectors” are a chronic condition and need to be dealt with by the user immediately.  Use SMART technology to regularly assess vulnerabilities and schedule system back-ups regularly.

Recovering data from hard drives that have failed as a result of excessive “grown” bad sectors involves the re-initialization of the service area modules and the reading and transfer of the data to reliable storage media using specialist test equipment.  This will make available what is referred to as a good file list and a bad file list. Bad files can be either corrupt or have minor damage yet still be use-able. Data Recovery customers are requested to check file lists thoroughly, open where possible and ascertain if files are of use.

Hard Disk Drive Bad Sector