For questions about defragmentation of file systems, the process of defragmentation or the effects of hard disk fragmentation.
Defragmentation is the process of rearranging blocks of data on a disk so that related blocks of data (e.g. all blocks of data in a certain file) are next to each other. File systems become fragmented as files are created, deleted and modified and blocks are written into the gaps. This slows down file access as the read head has to seek in order to find the next block. Defragmentation does not prevent this but it can remove fragmentation from a file system.