Disks become fragmented as files are written and deleted. Fragmentation tends to get worse over time. When you install programs on a new disk, the allocation units are written to a single, contiguous area. As you delete existing files and write new ones, free allocation units begin to appear all over the disk. Before you know it, pieces of the file for your new computer game are spread around like seeds in the wind, causing the drive head to dart all over the disk like a game of "Whack-a-Mole." Not only does this slow down the file transfer process, it also causes additional wear and tear on hard disk components, potentially shortening the life of the drive.
This simple illustration shows how fragmentation occurs. When you first load programs and copy data to your disk, the allocation units (in this case, ducks) are written one after another. All neat and tidy. Over time, however, writing, deleting and rewriting files to the disk leaves empty space in lots of places. The more deleting and rewriting the hard drive does, the more spread out the ducks get.
Fragmentation is unavoidable, although newer hard drives are designed to reduce its effects. The best way to avoid spreading allocation units all over the disk is to use a high capacity hard drive. If the amount of free space available remains high, files are more likely to be saved in contiguous areas of the disk. In most cases, a drive that is at less than 70 percent of its capacity will not benefit significantly from the defragmentation process. This is particularly true with newer, high-speed drives. These drives spin at a higher RPM and have faster read and write capabilities. Also, they often have larger buffers in which to combine file segments before sending a large file to the operating system.
Although technology and the lower cost of drive capacity have significantly reduced the amount and the effect of fragmentation, fragmentation still exists. Today's drives are larger, but so are today's files. Once you've loaded a disk with music, games, applications and even a couple of full-length movies, fragmentation can still be an issue.