How it happens

A disk drive creates and saves a file by laying it in contiguous clusters

on the disk drive. When that file is later read, the head in the disk drive

moves directly from one cluster to another on a single track. The head stays

in one place over that track and reads the file as the disk moves beneath

it. As more files are written to the disk, they are also laid out in contiguous

clusters.

When files are erased, their clusters are made available again as free

space. Eventually, some newly created files become larger than the remaining

contiguous free space, and those files are broken up and randomly placed

throughout the disk. As the file creation, editing and deleting process

continues, fragmentation becomes pronounced, exacting a progressively heavy

toll on system performance.