For those of you unfamiliar with the problem of file fragmentation, a quick—and quite simplified—primer is in order. Files are stored on a hard drive in blocks of data. The larger the file, the larger the number of blocks it is composed of. As your operating system accesses files, moves files around, and so on data blocks are not always arranged in the most effective manner. Imagine it like a messy office where as you opened file folders from your file cabinet you frequently placed documents from inside all over the room. You have a great memory, and you can find all the pages from each folder again if you need to, but you waste a lot of time just moving around looking for them.

Link:
http://lifehacker.com/5348638/five-best-disk-defragmenters

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