Monthly Archives: December 2018

Hard Drive Sector Format and Structure

The basic unit of data storage on a hard disk is the sector. The name “sector” comes from the mathematical term, which refers to a “pie-shaped” angular section of a circle, bounded on two sides by radii and the third by the perimeter of the circle. On a hard disk containing concentric circular tracks, that shape would define a sector of each track of the platter surface that it intercepted. This is what is called a sector in the hard disk world: a small segment along the length of a track. At one time, all hard disks had the same number of sectors per track, and in fact, the number of sectors in each track was fairly standard between models. Today’s advances have allowed the number of sectors per track (“SPT”) to vary significantly, as discussed here.

In the PC world, each sector of a hard disk can store 512 bytes of user data. (There are some disks where this number can be modified, but 512 is the standard, and found on virtually all hard drives by default.) Each sector, however, actually holds much more than 512 bytes of information. Additional bytes are needed for control structures and other information necessary to manage the drive, locate data and perform other “support functions”. The exact details of how a sector is structured depends on the drive model and manufacturer. However, the contents of a sector usually include the following general elements:

  • ID Information: Conventionally, space is left in each sector to identify the sector’s number and location. This is used for locating the sector on the disk. Also included in this area is status information about the sector. For example, a bit is commonly used to indicate if the sector has been marked defective and remapped.
  • Synchronization Fields: These are used internally by the drive controller to guide the read process.
  • Data: The actual data in the sector.
  • ECC: Error correcting code used to ensure data integrity.
  • Gaps: One or more “spacers” added as necessary to separate other areas of the sector, or provide time for the controller to process what it has read before reading more bits.

Note: In addition to the sectors, each containing the items above, space on each track is also used for servo information (on embedded servo drives, which is the design used by all modern units).

The amount of space taken up by each sector for overhead items is important, because the more bits used for “management”, the fewer overall that can be used for data. Therefore, hard disk manufacturers strive to reduce the amount of non-user-data information that must be stored on the disk. The term format efficiency refers to the percentage of bits on each disk that are used for data, as opposed to “other things”. The higher the format efficiency of a drive, the better (but don’t expect to find statistics on this for your favorite drive easy to find!)

One of the most important improvements in sector format was IBM’s creation of the No-ID Format in the mid-1990s. The idea behind this innovation is betrayed by the name: the ID fields are removed from the sector format. Instead of labeling each sector within the sector header itself, a format map is stored in memory and referenced when a sector must be located. This map also contains information about what sectors have been marked bad and relocated, where the sectors are relative to the location of servo information, and so on. Not only does this improve format efficiency, allowing up to 10% more data to be stored on the surface of each platter, it also improves performance. Since this critical positioning information is present in high-speed memory, it can be accessed much more quickly. “Detours” in chasing down remapped sectors are also eliminated.

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This is an archive of Charles M. Kozierok’s PCGuide ( which disappeared from the internet in 2018. We wanted to preserve Charles M. Kozierok’s knowledge about computers and are permanently hosting a selection of important pages from PCGuide.