Bobby installed a second internal drive for backup purposes, then he asked the Answer Line forum how come it doesn't show up in My Computer.
Chances are your new drive hasn't been formatted. This is easy to fix:
Select Start, then Run (in Vista, just click Start). Type diskmgmt.msc and press ENTER for the Disk Management tool. You'll find a square representing the new drive. Right-click it and select Format. It's all pretty obvious from there.
If you don't find the drive in Disk Management, you may need to set something in your hardware settings (AKA: your BIOS). Reboot your computer and look for a message (it should be one of the first things that comes up on your screen) saying something like 'Press KEY for setup'. Immediately press whatever key it tells you to, then search the menus for hard drive options.
Or the drive may not be properly installed. Open up your PC again and check the connections.
One last little comment: I don't recommend using an internal hard drive for backup. It leaves your backup vulnerable to many of the problems that you backup your data to protect yourself from. These include malware, fire, and simple theft.
I'd like to thank ARock84, crazy4laptops, and rgreen4 for their contributions to this discussion, which you will find at http://forums.pcworld.com/message/184947.