What Is a Sector?

A sector is a specifically sized division of a hard disk drive, optical disc, floppy disk, flash drive, or another kind of storage medium. It’s also referred to as a disk sector or, less commonly, a block.


Disk Sectors and Allocation Unit Size
When formatting a hard drive, you can set a custom Allocation Unit Size (AUS)—the smallest chunk of disk space the file system uses to store data. Common AUS options in Windows range from 512 bytes to 64 kilobytes.

For example, storing a 1 MB file on a drive with a 512-byte AUS will spread it across more sectors than on a drive with a 4096-byte AUS, simply because each block holds less data. If the file grows (say, to 5 MB), smaller AUS leads to more scattered file pieces, increasing fragmentation.

Larger AUS reduces fragmentation by keeping files in fewer, larger blocks, which usually sit closer together on the disk. This helps improve read/write speed and overall performance, especially for larger files.

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