Adding a second hard drive to Dimension 2400

I have read prior posts and I think I'm ready to try to add a second drive to my daughter's 2400. I have to get this right or I will be in the dog house. We bought a second drive bracket and are comfortable with the physical installation. I'm confused when I read about making sure the setting are in 'cable select'. Do I have to remove the original drive which will be the master and change its setting to cable select too or just the second drive? Thanks in advance for help.