Understanding of SG BUILD

Dear Forum,

I am facing problem with tape drive visibility on netbackup.

Total 8 drive should be visible but only 7 is getting detected on nbu.

On OS and OBP level i m getting it but not in netbackup.

I have tried rebuilding sg, rebooting media server etc.but no luck.

MY ? is : Need to understand -mt & -ml while giving for sg.build

Can you pls. explain or any good doc on this to understand it.

Thanks