A lot depends on what your D-drive is. On my computer, D is the restore drive and it's almost always better to leave it alone.
I try to keep my main data drive low use by archiving files I don't often need. Stuff I need occasionally I archive to zip files and keep them on the main data drive. Items I don't anticipate needing, I archive to a CD. (for instance, after my taxes are done I archive any data needed for them to a CD and put it in the folder with my tax return paper records.
I also search for any temp and recover files (they usually contain a ~ in the filename), and delete them.
Once I've archived and deleted everything I update my backup (I use SyncToy for this), then I run defrag on my main data drive. Then, if there are no problems with the defrag, I defrag my backup HD so it functions at its best, too.
I am not a computer professional, My advice comes from personal experience and/or friends who are computer professionals. Interested in Search and Rescue? Check out the Civil Air Patrol. Come Fold with us. TSF Folding@Home Team