You can put your dreams on a different drive, as do I, and it works fine. It does not copy anything.
Dream Manager is probably your problem. Use Deskscapes to manage your dreams and they can be stored anywhere you want. You simply tell Deskscapes where to look for them in settings.
Note: You'll then have to direct all new downloads to your chosen directory. They won't go there automatically. If you download and install a dream by the normal method, they will be placed in the default directory, public documents.
