1. A method performed by a computer system of storing a single-instance copy on a sequential storage medium such as magnetic tape, wherein the single instance copy is created from copies of original data objects, the method comprising: receiving or accessing multiple data objects from a computer network, wherein some of the multiple data objects are substantially identical; storing, on a random-access storage medium, a single-instance copy of the multiple data objects, wherein the single-instance copy contains a copy of one of the substantially identical data objects and not copies of the remaining substantially identical data objects, and wherein the random-access storage medium includes at least one reference to the copy of one of the substantially identical data objects; and providing for storage, on a sequential storage medium, the copy of one of the substantially identical data objects, and the at least one reference to the copy of the substantially identical data objects, in that order.