How to select only one drive when creating system image

I am trying to create a system image following this tutorial: http://www.tenforums.com/tutorials/5495-system-image-create-windows-10-a.html

but because I have 3 inteal drives, C, D, H when I try to create a system image , I get not enough space error. My drives show up similar to this screenshot but I can't de-select those I dont want.

How can I just select drive C to be backed up? they are all checked and grayed out.