A disk sector is part of a hard drive that is used to store data; it is effectively the smallest section of information saved on a drive. In the past, this was made up of 512 bytes of data, while newer systems have been developed to store more information. Each disk sector is part of a track, which is essentially a ring of data upon a disk, and these sectors can be grouped into clusters. The information saved for computer files and programs are stored on these sectors, along with data for identification and error correction.
To understand what a disk sector is, it is important to first know how a hard drive is designed. Within the physical case of a hard drive, there is a disk that is also referred to as the "platter." This is made up of a series of concentric rings, and each of these rings is called a track. The read/write head of the hard drive, which is used to read and write data onto the platter, is able to moved from the outer edge of the disk toward the center, and back again, to access each track on it.
A mathematical sector is part of a circle that can be visualized as a slice of pie. It consists of a wedge that extends out from the center to the edge of the circle, with two sides to form the shape of a pie slice. This type of mathematical sector can be applied to the circle of a hard drive platter, but this is not a "sector" as referred to in computer science. A disk sector is part of a particular track sectioned off based on the shape of that pie slice, not the entire slice or the entire track.
Each disk sector contains the same amount of data on a hard drive. In the past, the industry standard value for this was 512 bytes of information. Each byte consists of 8 bits, and a bit is either a "one" or a "zero," which are the basic components of all binary computer code. As data storage methods have improved, however, the size of a disk sector has expanded to 4096 bytes or 4 Kilobytes (KB).
Groups of sectors on a disk are often used to store data that is too large to fit on one disk sector, which is referred to as a cluster. The best way for information to be stored on a disk is as one continuous cluster on a single track, which can be read quickly and easily. This is not always possible, however, and so data can be written in different tracks and clusters. Such data is referred to as "fragmented;" "defragmenting" a hard drive is a process by which this information is rearranged to group it together more efficiently.
Along with the data stored in each disk sector on a hard drive, additional information is contained within them. This can include identifiers that are used by the system to determine where information is contained on the platter. There is also Error Correcting Code (ECC) that helps prevent corruption and maintain data integrity when errors occur.