A hard drive becomes "fragmented" as computer files are created, saved, updated and deleted within the same disk space. Software can "defragment" a heavily fragmented hard disk, which can boost your computer's performance.

How Files Fragment

Ideally, a computer file will be written to a hard disk in one big chunk. As files are deleted, however, gaps open up on the disk like missing puzzle pieces, creating small fragments of available space. A computer's operating system can save files to those fragments to fully use the space.

Disk Performance

Operating systems are designed to work fine with some file fragmentation, but heavy fragmentation -- which builds up with time and heavy use -- can result in slower "reads" and "writes" to the disk.

Defragmentation

The solution is to defragment the disk. Very simply, files are rewritten to the disk so they're contiguous -- one file is written completely, then the next, and so on, to avoid saving files in fragments.

Windows and Mac

Current versions of Windows (XP, Vista and 7) all have the Disk Defragmenter tool built in. Later versions of Mac OS X are designed to continually defragment the disk, so defragmentation is not necessary.

Manual Defragmentation

Prefer to do it yourself? Fully back up your hard disk, erase it completely, and then install and restore your operating system, applications and data files.

Warning

Always back up your data prior to defragmenting, if only to avoid data loss if there's a power outage or disk failure.

Source:

How Windows 7 Handles Defrag

Apple Support: About Disk Optimization with Mac OS X

How to Defragment Your Disk Drive Volumes in Windows XP