Quantum Superloader 3 LTO8 Tape Library work perfectly here.
Seem not even the kernel detects your device, so maybe something else is wrong. Already checked the cables?
A filename is not required, because we use the flag "O_TMPFILE".
What filesystem do you use? My guess is that the underlying filesystem does not support O_TMPFILE correctly.
That looks good. But the error "write failed - Medium Error" still look like a hardware error. Maybe you can test with another tape drive? Also Check if SCSI cables are OK ...