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. The controller
determines if the specified data storage drive is available, wherein if
the specified data storage drive is not available, the controller performs
failover processing by using a spare data storage drive in place of the
specified data storage drive.
