Hard disk is the slowest part of the computer compared to CPU and memory. If the contents of the data file are changed, added or removed several times then it is not necessary that the whole data file is located at the same place where it was accessed for the first time. The newly added file fills up the empty space and if the file is a big one then it looks for space elsewhere for its remaining portion. That is how the disk gets defragmented. Therefore, the accession takes more time every time. Disk defragging collects all the fragments of the file to combine into one. All the empty space is removed to make the whole disk contiguous.