I am trying to erase my Macintosh HD but I keep getting a error saying I can't unmount the disk. I am booting up from the install CD so IDK what the problem is
Exactly what BrianBaughn says : Check if you have a program still accessing it (anything from finder to 3rd party program to Time Machine encrypting your disk etc).
In activity monitor you can force close a program that can't be stopped FYI.