cool. That confirms that your drive does indeed have stuff on it. You (the user) don't see all files, windows hides its own stuff from you to make screwing it up more difficult.
First thing to check, go look at the Recycle Bin (if you don't see it do this
to see it). The files deleted usually go in there but are still physically on your drive (so you can undo a badly done delete). Delete them by right-clicking the recycle bin and selecting empty recycle bin. Or open it and select what files you want to delete send back the others in their original place.
The second thing to do is looking at the size of recycle bin's dedicated space,
on the desktop, right-click Recycle Bin, and then click Properties.
Under Recycle Bin Location, click the location of the Recycle Bin you want to change (likely your D drive).
Click Custom size, and then, in the Maximum size (MB) box, enter a maximum storage size (in megabytes) for the Recycle Bin. For most uses, more than a few GBs is overkill.
If this does not free up space, that windows is saving something of its things there. Let's find out.
Open Folder Options by clicking the Start button , clicking Control Panel, clicking Appearance and Personalization, and then clicking Folder Options.
Click the View tab.
Under Advanced settings, click Show hidden files and folders and untick the "hide system files and folders", and then click OK. (it should give you a warning that it's not a safe option, say OK, we just need it temporarily to see what is going on, afterwards you should undo this)
Now go in the D drive and see if there is anything new, post a screenshot of it if there is. Don't delete anything you see this way, we have to find out what it is first.