Contents of a sector usually include the following elements:

* ID Information: Within each sector a small space is left to identify the sector’s number and location, which is used to locate the sector on the disk and provide for status information about the sector itself. For example, a single bit is 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: Often referred to as spacers used to separate sector areas and provide time for the controller to process what it has been read before processing additional data.

* Servo Information: In addition to the sectors, each of which contain the items above, space on each track is allocated for servo information on drives that utilize embedded servo drives. Most, if not all, modern drives not employ servo technology.

Each of the elements identified above are collectively known as sector overhead. The amount of space taken up by this collective overhead is important, as the more bits that are relegated to disk or data management, the fewer that will be available for actual data storage. Hard disk manufacturers strive to reduce the amount of overhead information actually stored on the disk. When you see the term format efficiency, it refers to the percentage of bits on each disk that are used for data, as opposed to overhead. Obviously, the higher the format efficiency the better, however this information is closely held by disk manufacturers and extremely difficult to track down.

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