My HP Pavilion came with the "C" drive divided into two virtual drives, "C" and a "D" drive that the system uses for back-up. I keep getting error messages that my back-up drive is full. I have other back-ups via thumb drives and Carbonite and I want to scrub the "D" drive, but I cannot access it. How do I clean out unwanted back-up file to enable the PC to properly back-up?