As several posters have said, if your MDD Power Mac is the FireWire 800 model, you cannot boot off Mac OS 9. If you do have the FireWire 400 model, and it still isn't working, you might not have "OS 9" drivers installed on your HD. When initializing the hard drive, there is an option to install OS 9 drivers. If you do not select this option, you cannot boot off of the OS 9 CD. This happened to my iMac. I forgot to install the OS 9 drivers, and every time I tried to boot from the OS 9 CD, the computer would crash.