A storage and retrieval subsystem in a data processing system includes a plurality or magnetic tape cartridges, a host computer, and a library storage module storing the plurality of magnetic tape cartridges in substantially upright positions. The library storage module includes a first cylindrical array of storage cells centered about a vertical axis, a second cylindrical array concentrically arranged about the first array, and a library tape unit including a plurality of tape drives, each of the tape drives being adapted to receive one of the magnetic tape cartridges in a substantially horizontal transducing position. A robot within the library storage module is used to transfer selected ones of the magnetic tape cartridges between their substantially upright positions and the substantially horizontal transducing positions at a selected tape drive. A first controller outboard the channel communicating with the host computer is adapted to receive commands from the host computer for interfacing between the host computer and the library storage module, and a second controller inboard of the channel is adapted to receive commands from the outboard controller for interfacing between the outboard controller and the robot.