Try burning the Coriollis CD to a USB stick, and then boot from the USB:
- In Disk Utility, select the the Coriollis CD in the sidebar, then click the Restore button in the main part of the window.
- Drag the the Coriollis CD icon into the Source field on the right.
- Connect to your Mac the flash drive you want to use for your bootable Coriollis USB. The USB drive must be formatted with a GUID partition map.
- In Disk Utility, find this destination drive in the sidebar and then drag it into the Destination field on the right.
- Click Restore.
I have an external optical drive connected via Firewire.
My understanding is that some brands of external FireWire optical drives are not capable of booting a Mac, even though they can read and burn discs normally. You might try to get further info on your particular model.