Visually, imagine that your Hard drive consists of hundreds of thousands of little starage boxes, preserving the pieces of data. It either uses a storage box or it doesn't, never filling one with data from more than one program. When you add anything to your Hard Drive - a program, a note, audio, video, else - the record is not necessary saved in an area of contiguous disk space. For example, a 20 K file might be fragmented and spread over six disparate disk clusters, each 4 K in size. When this file is accessed, the system needs to obtain the contents of each of the clusters to open it, which would involve reading different areas of the disk for a single file. When you delete a file, the box is emptied, creating an empty space within your system. When defragmentation is performed, fragments of the same file are saved in contiguous areas of disk space, thus speeding up read access to the file and improving overall computer performance. There is a special section of the disk containing the File Allocation Table (FAT). It is a file system that keeps track of where all the files are stored on the hard drive. The FAT performance is significantly simplified after the HDD defragmentation.

I have already presented one of the best free portable defragmenters earlier:

http://freeware4review.blogspot.com/2008/01/jkdefrag-defragmeter.html

However, it is always good to have a choice. For your convenience, I have prepared the Web Rating List of 25 Free Defragmentation Computer Programs with brief description, review, ratings, and downloading links. I think, it is one the most complete listing of the programs in this category, as I worked hard to find them all.

http://www.rateitall.com/t-24840-hard-drive-defragmenters--freeware-reviews.aspx

Enjoy the Release!