Are you using Windows system image to back up,
by default, it should only include the drives required for Windows to run.
Try going to "computer"right click on OS drive/properties/tools/back up now.
In back up and restore select "change settings"/select destination drive/next/select "let me choose"/next.
Select the files you want to include in the back up,
you can also choose if you want to include a system image or not at the bottom of the page.