If you've taken a basic computer course at any point in your life, even high school, you likely learned about fragmentation as it relates to your files. Each file is saved in little fragments, or pieces, on the surface of your hard drive's numerous platters. It is said that this fragmentation leads to eventual system lag. A disk defrag performed once per month was said to make your system run more quickly and efficiently as it essentially makes the files more organized and closer together on the disk.
However, PC World conducted a test roughly nine years ago searching for the most efficient defragger. Surprisingly, it was discovered defragging the disk with any of the software programs made no difference at all in performance, even when measured against a fragmented disk. In 2001, Steve Gibson at Gibson Research admitted that modern drives don't really show signs of lagging due to fragmentation making them seemingly unnecessary. He adds that a defrag is still a good idea, however, to make data more easily recovered in the case of a system crash.
When NOT To Defrag
If you have a solid state drive (SSD), you might want to think twice before defragging. These drives can wear out over time due to excessive writing. The drive will read until the cows come home, but writing will shorten its life span. Because the defragging process does a high amount of writing, it is not recommended you defrag SSDs much. You should still periodically defrag, probably about once a year.
If you are running Windows 7, you'll be pleased to know the OS takes care of the defrag for you. Windows 7 features a built-in disk defragmenter that is scheduled to run itself automatically, about once per week. You don't have to worry about going into the Control Panel and setting the process up as it is already set to occur by default. You've been blissfully unaware that your computer has been defragging itself for as long as you've owned it.
Own a Mac running OS X 10.2 or later? There's no need whatsoever to defrag. There are safeguards built in to the OS, keeping files from being stored in any fragments whatsoever. OS X's HFS+ file system searches for larger blocks of free space rather than space that has recently become free, thereby assuring the files aren't fragmented at all. OS X gathers small files that are in groups and automatically combines them in larger free space areas of the disk, defragging all of the group files. If you open a file that is made up of more than 8 fragments, OS X automatically defrags the file.
When To Defrag
For any hard drive that is not a SSD, experts recommend defragging approximately once per month. Experts also point out fancy software isn't required: Windows' defragger will work nicely. There is no noticeable difference between the performance of Windows' defragger and that of a third-party defragger.
You should defrag your external drive manually at least once a month if you are using it often and if it isn't hooked up to your computer as it probably isn't being automatically defragged by the automatic defragger.
Lastly, Mac users, you aren't off the hook yet. You should perform a defrag if you reach 10% or less of free space. With this small amount of space, it becomes impossible for OS X to perform the auto defrags it normally does, so it's time to either increase your storage space by expanding or clean house and delete some files or store them on an external drive.