Bleach drive

I have Norton utilities 16 (Registered).

I am trying to bleach 1 specific disk but when I check only that drive or any other specific drive for that matter the 'clean' option is grayed out. 

 

All it will allow to work is the 'clean all' option.

 

Why is this?

 

I am not willing to risk this option on my main operating drive, only one of the slave drives.