I can vouch for possibly the keyboard being the issue. The last Sawtooth (converted to Gigabit Ethernet) that I used would always eject its drive on startup and would never boot from an optical disk provided I had a standard Windows labelled USB keyboard connected. I assume it's due to differences in the keyboard circuitry between standard Apple keyboards and PC keyboards related to the Eject key.

May be worth checking out if you have a standard PC keyboard.