You shouldnt try to unload sg or sg modules while NBU is in use or you could jepordize the backups; because you just yanked the underlying tape driver NBU needs to do tape io commands.
But since none of the local drives are being used, it might be possible to termiante ltid, then attempt to unload sg, and st driver and reload time. However, there are some risks, as I've heard from some users on rare occasional leading to kernel panics. This would definitely kill that long runing backup. If it's that important let that job finish before doing anything.
If you have sg3_utils installed, and lsscsi installed, you can check and confirm device files. Or check tpautoconf -t (under /usr/openv/volmgr/bin)