Check this out: http://ultradefrag.sourceforge.net/en/index.html?download http://ultradefrag.sourceforge.net/en/index.html?download Specifically, scroll down t

Check this out: http://ultradefrag.sourceforge.net/en/index.html?download http://ultradefrag.sourceforge.net/en/index.html?download Specifically, scroll down to the "PDF Documentation" section, and open that. Then, search for the word "algorithm". They give a high level overview of how they perform the defrag.