Media library with failover capability

A digital data storage unit comprising:

• a multiplicity of storage media slots for receiving storage media units,
• a plurality of storage media units loaded in particular ones of the storage media slots,
• a plurality of data storage drives electrically connected to the digital data storage unit, each data storage drive having a unique logical drive address, the data storage drives connected to at least a host computer via a bus structure, wherein one or more of the data storage drives are reserved as spare data storage drives,
• a loader mechanism for selectively moving a storage media unit between a storage media slot and one of the plurality of data storage drives, and
• a storage unit controller connected to the host computer, the storage unit controller configured for:
• receiving and decoding a host command sent by the host computer at the storage unit controller, the host command including a source address corresponding to a storage media slot location, and a logical destination drive address corresponding to a data storage drive specified by the host computer, and
• determining if the specified data storage drive is available, and if not, then using a spare data storage drive instead of the specified data storage drive by moving the media storage unit to the spare data storage drive.

A digital data storage unit comprising:

• a multiplicity of storage media slots for receiving tape cartridges, each tape cartridge including at least one spool of magnetic recording tape,
• at least a plurality of tape cartridges loaded in particular ones of the storage media slots,
• a plurality of tape drives electrically connected to the digital data storage unit, each tape drive having a unique logical drive address, the data storage drives electrically connected to at least a host computer via a bus structure, wherein one or more of the data storage drives are reserved as spare data storage drives,
• a loader mechanism for selectively moving a tape cartridge between a storage media slot and one of the tape drives,
• a storage unit controller connected to the host computer, the storage unit controller comprising:
• a storage controller configured for:
• masking one or more of the tape drives from the host computer as spare tape drives, whereby the spare tape drives are not directly accessible by the host computer,
• receiving and decoding a slot-to-logical tape drive exchange command including a logical drive address for a specified tape drive and location of a storage media slot holding a requested tape cartridge, sent by the host computer,
• determining if the specified data storage drive is available, and if not, using a spare tape drive in place of the specified destination tape drive, and
• a loader controller responsive to the storage controller for operating the loader mechanism to move the requested tape cartridge from the storage media slot location to the spare tape drive and for loading the requested tape cartridge into the said spare tape drive.