I have a D7100 I got at Christmas. I am having a new problem. Twice in a row when I downloaded, half my photos were missing. Trying to figure out what is happening. Anyone had something similar happen? Is it as simple as a bad memory card? Why didn't they just record on the backup card then?
I would re-format both cards, in camera, using the lcd menu. Then take some test pictures; then use the Nikon jumper USB cable that came with your camera and plug it into both the camera and your usb port on your computer. Then turn the camera on. Windows should come up automatically with a file manager. If not, use Windows Explorer to view them; See if you see your photos then. If not, it's probably a bad card.
If it is a suspected bad card, I would contact the card manufacturer and see what they suggest. There is 'recovery' software available from most card companies, but there may not be anything to recover.
Let us know how it goes.