Fragmentation is caused by creating and deleting files and folders, installing new software, and downloading files from the Internet. Computers do not necessarily save an entire file or folder in a single space on a disk; they're saved in the first available space. After a large portion of a disk has been used, most of the subsequent files and folders are saved in pieces across the volume.

When you delete files or folders, the empty spaces left behind are filled in randomly as you store new ones. This is how fragmentation occurs. The more fragmented the volume is, the slower the computer's file input and output performance will be.

Defragmentation is the process of rewriting non-contiguous parts of a file to contiguous sectors on a disk for the purpose of increasing data access and retrieval speeds. Because FAT and NTFS disks can deteriorate and become badly fragmented over time, defragmentation is vital for optimal system performance.