I have the same set-up on both my wife's and my own laptops.
I use Drive D all the time for all my own files as well as downloads from the internet.
I don't use the 'My Documents' folder (on drive C) at all. The only new files that have gone onto my C drive are additional programs that I have installed.
I have no problem accessing any of my data on drive D.
I would suggest you open up two windows, One for Drive D and one for your 'My Documents' on C. Highlight all the contents and drag them across. Ascertain that they have successfully transfered and then delete the original content from your 'My Documents'. Do the same with your pictures and music. You can always make shortcuts and place those in your old folders.