Why won't my new IDE hard drive work after I just installed it?
Answer: IDE drives must be set up in a "Master-Slave" configuration. This means one hard drive is designated as the master device, and the other one is called the slave drive. The drive with your operating system installed on it should be the master, and the CD or other hard drive should be the slave.
To determine which drive is which, you usually have to change the jumpers on the back of the drives. (They are typically little plastic pieces that go on to small metal pins). You'll need to check the manuals that go with the drives to know how to set them correctly. An improper setup of your devices could likely result in one or both drives not functioning.
Note: You can have one master device per IDE controller. So if you have two IDE controllers on your motherboard, you can have two master devices. Then you don't have to worry about any master-slave issues at all. Yea!