I bought another (internal) hard drive and was trying to install it. It showed up fine in Windows Device Manager but would not show up in My Computer (with a drive letter)
When you buy a HD it has nothing on it so it will _not_ show up in the My Computer. Yout must use the Windows Disk Management utility (Start -> Control Panel -> Administrative tools -> Computer Managment -> Disk Management) to first partition the HD to give it a drive letter. Then it can be formatted.