Disk Optimizer Not Doing C Drive?

I am running Win 7 Home.

I am trying to use Norton Security to optimize my C Drive, that main drive that comes with nearly all PCs. When I run the Optimizer, though, it is doing only two external hard drives attached to the computer by USB, not the internal C drive. I am trying to find the settings to change this but I can't. I scoured the admin setting under Settings and nothing is popping out at me re adding the C drive to optimization process, and removing the others.

I looked through the manual and am finding nothing about how to do this.

Can anyone help?