With a lot of help from mph999 and Marianne it has been decided that the fault or error is definitely coming from the library itself.
This is demonstrated in the robot log:
17:22:45.372 [2700.1124] <5> tldcd:move_medium: TLD(0) initiating MOVE_MEDIUM from addr 16 to addr 4139
17:22:48.043 [2700.1124] <3> tldcd:log_scsi_error: TLD(0) key = 0x5, asc = 0x53, ascq = 0x81, ATTEMPTED TO EXTEND EMPTY ENTRY/EXIT PORT
17:22:48.043 [2700.1124] <3> tldcd:move_medium: TLD(0) Move_medium error
17:22:48.043 [2700.1124] <5> tldcd:tld_main: TLD(0) closing/unlocking robotic path
17:22:48.043 [2700.1124] <3> tldcd:ChildExit: TLD Child process 2700 has terminated abnormally: Cannot move from media access port to slot
17:22:48.965 [2392.7552] <6> tldcd:check_unit_attention: command_init to check for UA on robot 0
17:36:19.492 [6580.3276] <5> tldcd:move_medium: TLD(0) initiating MOVE_MEDIUM from addr 19 to addr 4176
17:36:21.805 [6580.3276] <3> tldcd:log_scsi_error: TLD(0) key = 0x5, asc = 0x53, ascq = 0x81, ATTEMPTED TO EXTEND EMPTY ENTRY/EXIT PORT
17:36:21.805 [6580.3276] <3> tldcd:move_medium: TLD(0) Move_medium error
17:36:21.805 [6580.3276] <5> tldcd:tld_main: TLD(0) closing/unlocking robotic path
17:36:21.820 [6580.3276] <3> tldcd:ChildExit: TLD Child process 6580 has terminated abnormally: Cannot move from media access port to slot
17:38:34.953 [7356.3332] <5> tldcd:move_medium: TLD(0) initiating MOVE_MEDIUM from addr 20 to addr 4177
17:38:37.250 [7356.3332] <3> tldcd:log_scsi_error: TLD(0) key = 0x5, asc = 0x53, ascq = 0x81, ATTEMPTED TO EXTEND EMPTY ENTRY/EXIT PORT
17:38:37.250 [7356.3332] <3> tldcd:move_medium: TLD(0) Move_medium error
17:38:37.250 [7356.3332] <5> tldcd:tld_main: TLD(0) closing/unlocking robotic path
17:38:37.250 [7356.3332] <3> tldcd:ChildExit: TLD Child process 7356 has terminated abnormally: Cannot move from media access port to slot
With the library error message specifically being
key = 0x5, asc = 0x53, ascq = 0x81, ATTEMPTED TO EXTEND EMPTY ENTRY/EXIT PORT
Martin offered the following as well,
To get the exact meaning, you will probably have to ask the library vendor, as it is their hardware that is producing the error.
I'll have another look, but I didn't find an exact match in out TNs.
However, I can confirm ASC/ ASCQ errors are sent from the hardware, impossible for NBU to generate this error.
....
Key Code Qualifier is an error-code returned by a SCSI device.
When a SCSI target device returns a check condition in response to a command, the initiator usually then issues a SCSI Request Sense command. This process is part of a SCSI protocol called Contingent Allegiance Condition. The target will respond to the Request Sense command with a set of SCSI sense data which includes three fields giving increasing levels of detail about the error
The vendor Quantum has not really offered an explanation yet but is sending me a bucket load of parts and an engineer to investigate further.
I'm marking this as solved as we have concluded that the fault is not with Netbackup
Thank you everyone for you help
Katie