Abstract: A sealed library type optical disc unit is combined with an exchangeable cartridge type disc unit in a data storage and retrieval system. The optical discs within the sealed unit are exposed, as opposed to being encased in cartridges. A data access unit can quickly and efficiently access the exposed optical discs for loading them on an optical disc recording/reproducing unit. A control unit manages the transfer of data from a host computer to the storage and retrieval system. If a data write request exceeds the storage capacity of the optical disc within the sealed space, the excess data is written to the cartridge type disc unit without intervention by an operator and without interruption of the post processor. During a read operation, if data is requested that has been previously recorded on a cartridge type disc, the disc ID is displayed so that an operator can insert the disc into the cartridge type disc unit for the read operation.