Abstract: A relatively simple and easily installed transfer unit is disclosed employing a movable cartridge bin assembly which travels horizontally between two or more cabinets of respective tape cartridge library systems, to enable the expansion of individual library systems thereby increasing overall mass storage system capacity. The bin assembly is presented to a typical cartridge handling robot of respective library systems as if it were part of the stationary bin array housed within the cabinet, which allows the cartridge to be accessed and loaded in a constant orientation. A drive means provides the translation of the bin assembly and cartridge, and depots at preset stop locations include sensor means for indicating the location of the bin assembly and the presence of a cartridge in the bin assembly.