Sorry for asking you to repeat yourself. Once enough goes back/forth I lose track
Here is an export of the keys in question from my registry. I'm not suggesting you directly edit these. These were set by installs long before S-Plat and CCC ... CCC automatically picked up the ContentDir key. Sonar loops and other content have continued to install here, Dim Pro, Rapture and Rapture Pro have also honored this.
I mention this because C3 might be [silently] refusing to honor your change request to keep from mucking up other products, or it may simply not have rights to change this key.
=============================================
[HKEY_LOCAL_MACHINE\SOFTWARE\Cakewalk Music Software]
"ProjectDir"="C:\\Cakewalk Projects"
"ContentDir"="D:\\Cakewalk Content"
"Language"="English"
"SampleDataDir"="D:\\Cakewalk Content\\Cakewalk Sample Data"
=============================================
If you do decide to try editing this, I would copy everything in the ContenDir on C to the new one, back up your registry, and rename the original ContentDir. Then edit the ContentDir key to match your new location. This should make it easy to go back if it doesn't work.
THEN startup CCC and see if it picks up the new location, and if not, maybe it will allow it to be changed.
Be advised I am NOT a GURU. If you do this, you assume all risk. Evaluate what I have written here and make your own decision on whether or not you want to try this.