Customer expandable modular rack mount library using expansion module units having features to join access doors and align components between units

An expandable storage library for storing digital storage media, such as storage tapes or disks is provided. The library includes a base unit and robotic system to which additional expansion units may be added. Several expansion units may be installed in the library with the base unit so that the robotic system can access media storage racks in any of the expansion units or in the base unit. Doors on the base unit and expansion unit may be interlocked together to function as a single door. A Z pole guide on the base unit may be extended by adding expansion guide poles that guide the movement of the robotic system through the library.