Brand new iMac wont boot from anything but HD?

Got a brand spanking new 20" imac here that ive been preparing an image on. Wanted to boot off my firewire drive to take said image, I hold down option, select the boot volume, the disk clicks a bit, screen goes black, then grey, then it boots off the internal HD.

Thought it may well be the drive or the partition is screwed or something so went to reinstall OSX on that drive, but it wont boot from DVD either! Does the same, accesses the disc for a second or so, screen blanks, boots to HD. I've tried this with and without the external HD connected.

Why cant I boot off anything?!

Is this disc the one that came with the machine. I found that the new Nvidia education imacs cannot boot off of a DVD that is lower than 10.5.6. So to clarify if the disc you are using is 10.5.0 - 10.5.5 retail or other then they won't boot due to the graphics card drivers not being part of those builds.

Possibly the same thing with your external HDD. You may need to update the version you have to 10.5.6 and above.

Ahh, that could well be it then. I was using a 10.5.1 DVD and I think the installs on the hard disk are 10.5.4....should be able to update those on an older machine though. Thanks :)

Worked a treat, upgraded using a first gen intel imac and it boots fine now. Thanks again