How do you use two hard drives in one computer?

In a desktop computer, open the case. Connect an IDE cable to the computer, and connect the other end of it to the second hard drive. The connection only works one way. As soon as you turn the computer on, if both hard drives don't sound like they are working, then you make a mistake with the connection, and you basically have to flip the cable end 180 degrees and plug it back in. Now enter your BIOS setup, and it should be able to detect the drive. When you enter Dos/Windows, it will make your new hard drive drive D, if the drive you normally use has only one partition.