I have a Rev2 iMac (originally 266 Mhz) that I have upgraded to a 500 Mhz G3 using a Sonnet board. I used to be able to boot from a replacement CDRW drive, but now I cannot do so. Here are the symptoms:

When I name the CD drive as the startup drive and place a bootable CD (MacOS 9.0) in the drive, the system does not see the drive and instead boots from the HD. At some point in the boot-up cycle I get the message "This computer cannot read this disk" (Eject, Initialize, ...). When I select "Eject" and immediately reinsert the CD, it opens and displays the CD's contents immediately.

I have also tried booting with the "c" key pressed as well as using the Shift-Option-Cmd-Delete combination. Same result as above.

My firmware is up to date (1.2). The CD driver is up to date (2.0).

I need to boot from an alternate source to fix an HD problem, but Rev 2 iMacs cannot boot from USB external disks. I must somehow get the machine to boot from the internal CD drive.