Files are stored on disk in chunks called clusters. Clusters are of a fixed size, so the larger the file, the more clusters required to store it.
Because files are stored in chunks, and because files are always being created and deleted, it is not always possible for the operating system to store files with all the chunks arranged consecutively. When a file's clusters are in a consecutive row it is said to be contiguous; if not, it is fragmented. A file could be in 2 fragments or thousands of fragments depending on the size of the file and the layout of the disk.
When a file is in more than one fragment, the disk drive takes longer to read the file as the drive heads must read from one section of the drive, then move to another section, then to another and so on. Drives can seek to new locations in a matter of milliseconds, so this is not noticeable for a single file, but when a drive has hundreds or thousands of fragmented files the cumulative effect is considerable.
Fortunately, Windows 2000 and later have an in-built defragmentation program. It will rearrange the files so that as many as possible are stored contiguously. It will also try to ensure free space is in one big block.
Depending on the existing fragmentation of the drive, this could take 5 minutes or several hours. So defragmenting a drive is a good task to set running during lunch or overnight (turn the monitor off).
Most systems will “perk up” noticeably after defragmentation, especially if it has not been done for a long time. Generally speaking, once a week is a reasonable frequency.
To defragment a drive:
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