You might save yourself a lot of bovver if...
... your 'D' drive genuinely is empty, as you state in your post. You have by now ticked the 'show hidden' box, yes? And you have confirmed by clicking the 'properties' of the drive that it has nothing on it.
So, what has filled up your 'C' drive? If it's stuff like movies etc, it doesn't matter where you put them, provided you remember - and tell your viewer next time you want to look at 'em. Same applies to most other documents: photographs & the like which can take an awful lot of space over the years.
Copy the lot across, check they've copied cleanly, and then free up that space on 'C'.
Any good?