Using Disk Utility to Erase Disk-won't unmount

I have a MacPro and 4 drives inside it, one of which I need/want to erase. I have backed it up and using DU tried to erase it = by both highlighting the drive identifier and the name - in both cases, whether I try to erase disk or set it to erase to one partition, I get a message saying it can't do it because it can't unmount the disk.

I backed up my startup drive to this drive so I could wipe and reinstall OSX on my new SU drive but now I can't delete items off the backup drive [the one I want to erase] because the message says I can't delete backup items. And I can't erase the drive to get a brand new drive to work with.

I don't need the old SU material on this drive, it was just a way to be sure I had it before using Migration Asst to reinstall the SU on the new SU drive = I used SuperDuper to clone the old SU onto this drive - then I used Migration Asst to move the stuff back to the new startup drive = but now I want to EITHER get rid of the system files and apps [no need for those = I don't min