Disk-to-disk (D2D) is a computer data backup scheme that involves moving information from one physical disk to another, with both the source and backup hardware usually being hard disks. Using a hard disk to backup data from a computer system is distinctly different than sometimes more traditional types of backup architectures, such as using a tape backup or optical storage medium. One of the largest differences between disk-to-disk backup and other forms of long-term data storage is that the backup disk generally is in the same format as the source disk, meaning problems that could occur from transitioning from one medium to another, and problems that can arise from deconstructing an entire file system, are not necessarily present. Another feature that can make disk-to-disk backup desirable is that a hard disk allows for non-sequential, or random, access to its contents, unlike other media that can force a system to walk linearly through all the data that has been saved to find a single file or record.
When dealing with the backup and long-term storage of computer information that is sensitive or valuable, tape storage systems have been one of the most traditional formats available. This is largely because the tapes are stable, have a low occurrence of failure and are largely a physical storage medium that, in and of itself, does not rely on mechanisms stored on the tape for retrieval. Some of the drawbacks of using tape as a backup medium are that the tape only supports sequential and linear access to data, data have to be translated into the format the tape supports, and the tape itself must be physically transported for duplication of the tape to occur.
Using a disk-to-disk backup system mitigates some of the complications that a tape-based backup might entail. If a hard disk is being backed up to another hard disk, then one of the easiest methods is to use disk cloning techniques to create an exact duplicate of the source disk. A hard disk also allows random access to information it contains so recovery of just a certain file or directory can be performed quickly and easily. Unlike tape, data does not have to be translated into a separate format for storage, meaning a backup disk can be used to perform a disk-to-disk backup across a network to a remote offsite location.
One of the largest potential disadvantages of using disk-to-disk backup is that the hardware used in a hard disk has a higher rate of failure and errors than a tape system. A hard disk also is not an independent physical storage medium and relies on electronic hardware and other supporting mechanisms to write and retrieve data. Many of the problems with a disk-to-disk backup scheme, however, can be mitigated through the use of redundant hard disks for backup. Some systems also include tape backups in addition to a disk-to-disk backup to ensure that there is no chance of catastrophic data loss.