The Downloads and Desktop folders look pretty fat. The contents of Downloads are good candidates for review. Do the files need to be stored on C or can they be archived on the external drive? My guess is that they can be archived if they are retained at all. Often times, companies use the Desktop as a convenient temporary space during installation but leave the files for the user to clean up. I suspect there might be some redundant files in your Desktop folder.
You could also move all the projects which are not active to the external drive and copy them back when needed. This is pretty easy with per-project audio folders.
Cakewalk Content could give up a couple of GB pretty quick by deleting/moving the Tutorial folders (unless you use them). If you do not work with audio loops, there are at least 4GB of audio loops usually installed in Cakewalk Content.