How do I get it to back up drive D?

WD External Drives WD Portable Drives
1

I used MY Passport and it automatically backed up drive C.–Great.

Now how do I get it to back up Drive D?

2

Are you using WD SmartWare to back up?

If yes, WD SmartWare should have drop down of drive to pick from C: or D:

Pick D and Click Back up Drive.