defragmentation 
Hard disks tend to break files into pieces, with each piece stored in the nearest location available as the file is saved. Over time, files can become fragmented all over the hard disk, causing the drive to slow down as it reassembles those pieces when an application wants to load a file into memory. A defragmentation utility (such as Windows' Disk Defragmenter) unscrambles the files, rewriting them contiguously onto the hard disk for quick access.