Robotic Library Error when doing an inventory on an empty slot

Any time an error occurs when running an inventory, you should open a support case with the hardware manufacturer. The backup software only sends commands to the device to pull a tape from the slot, put it in the tape drive, read the header information on it, and return it to the slot. If the device goes offline, then it is usually the hardware that terminates the connection to the backup software and/or OS if something is not configured properly on the library or if the device is defective.