As I previously posted, if you type shell:sendto in the start search box it will list the sendto folder. Open the folder. Open 'computer' and drag drive D to the sendto folder. Drive D shortcut will be listed in the right click menu under send to. Using this method you will have duplicate files on C and D. Not a bad idea as you can then check your files on D to make sure there has been no corruption, when copying over, to D. You can then delete them on C.
If you use the system to open drive D in computer, then, instead of copy and paste, use cut and paste. This will remove the files from C and move them to D. Unfortunately, with this method, if the copied files become corrupted (it can happen) then you won't have any backup on the C drive.