How do I set up which disks are optimized?

I am once again very frustrated with Norton 360 (ver. 20 something) as I have searched but cannot find any way to set up which hard drives are optimized.  It seems my only options are Do It When Scheduled or Do It Now, but no way to tell it which disks to do!  I have spent over an hour going in, out and all through all the menus.  I have a C drive which seems to be on the schedule but I also have an external D drive (USB backup drive) which I would also like it to check.  How do I find the place in Norton 360 where I can do this?  Thanks!