Disk-to-Disk backup, sometimes referred to as “D2D”, refers to the method of making a backup (of a hard disk or files on a disk). What has been “normal” for years is making a backup to some kind of tape, or perhaps some other type of media (like a CD or DVD).
The main benefit of this method? Speed! When information is moved between to disk drives, it is much faster than having to deal with tapes or tape libraries. This is because tapes usually have a limited capacity and have to be swapped in and out as the backup takes place, and as a result, tape backups are usually limited to times when the computer system that is being backed up is not very busy. Now that many businesses, and the computers that drive them, run 24 hours a day, the time to make backups has become very limited and the faster you can make your backup, the better.
Another advantage with using D2D is that if and when some data has to be recovered, it can be done much faster. This is because disk drives are “random access” devices (unlike tapes) and just the portion of the data that you want to recover can be found quickly instead of starting at the beginning of a tape and having to find that same information by reading from the beginning to the end of the tape. Even if the tape is indexed and the drive can “fast-forward” to the correct record, it is still faster than having the backup on a disk.