Abstract: A peripheral data storage hierarchy includes three storage levels. The top storage level is a fast accessing direct access storage device(s), such as magnetic disk drives. The intermediate level is an automatic warehouse type library, storing a large plurality of optical disks, which are automatically transferred between storage cells of the library and optical disk drives operatively connected to the host processor. The bottom level of the storage hierarchy includes one or more stand-alone optical disk drives and a shelf unit. Personnel manually carry the optical disks between the stand-alone drives and the shelf unit upon mount and demount commands received from the host processor. The intermediate library level uses the same type of optical disk as used in the bottom level of the data storage hierarchy. An I/O station in each of the automatic libraries enables manual access to the optical disk for transferring the optical disks between the intermediate and bottom levels.