Your pc will not automatically start using the next disk (would be nice if it did though !).
What you need to do is manually transfer some of your files over.
I store all my music and videos on my second drive, and install games onto it instead of the deafult c:\program files\whatever.
Cable select / slave doesnt make much difference, although some drives wont work with some settings, so just give it a try and see which works. It wont damage it in any way