finally read what I was really afraid of reading all this time :

" Turns out the Mirror Door/Firewire 800 model G4's that cannot boot into OS9, cannot actually boot from a factory Jaguar CD either! It would only boot with the DVD that came with the computer. I thought I would be crafty and swap the hard drive out with a brand new MDD model I just got in. (DUAL 1.25mhz). Made things even worse then, the display went wacky!

AppleCare Tech support told me that the my Firewire800/MDD machine would ONLY work with the hard drive that came with it. "What happens if that hard goes out?", I asked. "we replace it!", was the tech's answer. "What if I want to put a bigger drive in, replacing the original drive?", I asked. "Can't do it", was his answer.

Apparently you can add 3 additional drives (without adding another adapter card), but you cannot replace the original drive with anything, but what you would get as a replacement from APPLE.

The technician told me that it has to do with the firmware Apple is using in these models. "To prevent you from booting into OS9", I joked. He responded quite seriously, "That's about it!""

and I bought this mac cause apple had finally changed. bastards!