The data storage hierarchy of IBM System/370 compatible mainframes can be augmented by a plug-compatible optical disk storage system. The storage hierarchy consists of main memory, rotating magnetic disk memory or Direct Access Storage Devices (DASD), magnetic tapes, and hardcopy. Only data stored on DASD is available for on-line processing. Data recorded on magnetic tape is generally off-line. Long time delays are generally associated with the transfer of data from the tape library. The need exists for a form of high-capacity on-line storage. Optical storage can satisfy this need, and by emulating 3480 cartridge tape subsystems, the benefits of Write-Once-Read-Many (WORM) optical disk storage can be made conveniently available to IBM and compatible mainframes. To accomplish this, a controller is required which attaches to IBM's I/O channel and emulates a 3480 tape control unit. The resulting optical mass storage system is compatible with IBM operating systems and applications software. Data is stored on the optical disk as a collection of virtual tapes. Virtual tapes can be retrieved from a selected optical disk and, after processing, the revised data can be written to unused sectors of the optical disk. The virtual tapes save the time expended in tape mount/demount operations. The optical disk system is able to interpret and act on mount/demount messages directly, providing fully automated operation. In a jukebox configuration, thousands of virtual tapes can be handled automatically with no operator intervention. The described optical storage system addresses a longstanding requirement for on-line access to large volumes of data. By emulating the IBM 3480 magnetic tape system and storing data as virtual tapes, it offers a practical solution to the need for access to terabytes of data.