défragmentation

CONT: When you use the same file over and over again, writing, rewriting, saving, and deleting parts of it on the same disk, the file becomes fragmented. That means that although you can't tell, your operating system is storing all the data from that file as separate packages of information, distributed on different parts of the disk. Although fragmentation does not lose the information contained in the file, it does eventually slow down access to the file itself, because the OS must search the whole disk to create the sum of the file's parts. Defragmentation collects all those parts into one stream of data again, speeding up your system.