when the computer is booting, press the <Command> and <~> keys at the same time. This will bring you to a firmware monitor screen.
next, type p to look at the boot params, then I think the boot device is listed there, it can typically be 'sd' or scsi disc or 'en' ethernet. You want to make sure the boot device is set to sd.
press the power button to turn off and try to boot again, hopefully it will boot from the first scsi disc which should be your internal HD.
hope this works for you.
rp