I'm having exactly the same problem.
This is my Windows Disk Manager Report:
The C drive seems to be reporting correctly - I moved a ton of stuff out of D and into C when this first started.
D is my recovery partition - it's fine too
But my Data Drive (Q) is reporting as full when it is less than half full.
At this point the only think I can think of is to back up all the files and reformat the disk. At least it is a data drive.
I'd love to figure out what is happening.
My tech level is moderate - so many of the suggestions you have made in this thread are out of my range.