Your thought is correct, although it is possible
Microsoft says this is only suited to a test environment.
Your desktop is part of your profile including your Documents, videos, Downloads etc and they are not designed to be moved. My solution would be to mount the new volume so you have more space, this would enable the drive to be under the same drive letter and also appear that your Primary partition is extended.
Edit: please excuse, this reply is talking about the complete user profile, not single user docs.