computer help: Computer Issues! - Help.com

no it to defrag is like tidying up, … putting the crap all over your floor in the drawers where they belong :)

The disk can get fragmented because when you have many files on there, and over time you delete many files, then there are gaps left. These gaps may be too small to hold a new file you are saving to disk. So the computer chops up the file into smaller sections to fit into the various gaps. This results in a large file being spread over the disk in several sections, or depending on how fragmented the disk is, the new file could be in many sections, all of which have to be reassembled when the file is accessed again. This takes a lot longer to retrieve the file.

When a disk is defragmented, all of the files are moved down next to each other so that there are no gaps, and all of the gaps now become one large open space. Now large files can be stored in one piece and in one place, until such time that things get untidy again after many other files have been deleted causing new gaps to form.