Abstract: A process/method for controlling a digital data storage unit including a multiplicity of storage media slots for receiving media storage units, a plurality of media storage units loaded in some of the storage media slots, a plurality of data storage drives each having a unique drive address, a loader mechanism for selectively moving a media storage unit between a storage media slot and one of the plurality of data storage drives, and a storage unit controller connected to at least one host computer. One or more of the data storage drives are reserved as spare data storage drives wherein the spare data storage drives are masked from the host computer such the spare data storage drives are not directly accessible by the host computer. The storage unit controller receives and decodes host commands including a source address corresponding to a storage media slot location, and a destination address corresponding to a data storage drive specified by the host computer.