Hello Jason, and welcome to Seven Forums.
If you just wanted to use the D: drive for data and didn't care about losing the OS on it, then you could move any files that you still want to keep from D to C temporarily, format D, copy back the data to D, and be good to go with a lot more free space as well.
Otherwise, you could use the free program Unlocker
to delete the hidden
protected OS D:\hiberfil.sys
file if you were not able to just delete it normally.
Hope this helps,