Short for disk-to-disk, a type of data storage backup in which the data is copied from a disk—typically a hard disk—to another disk—such as another hard disk or other disk storage medium. In a D2D system, the disk that the data is being copied from typically is referred to as the primary disk and the disk that the data is copied to typically is called the secondary or backup disk.
Contrast with D2T. One advantage of a D2D system over a D2T system is that backup files can be accessed directly just like any disk system. Tapes, conversely, have to be searched linearly and are not as expedient at retrieving data.