Huh, well I just learned something. I never knew you could specify the device like that on boot. That's very handy to know, although I now have two scsi burners that allow boot another machine I bought randomly had one and it also allows me to boot from it although it's generic and unbranded so I've no idea any details about it. But the yamaha ones will probably find there way to another owner someone who has more optical drive bays perhaps.If you are still having issues with this, the following may help:
At issue is the fact that the Yamaha drive isn't Apple ROM'ed, so it's easy to say it isn't bootable (as Yamaha support used to do without much thought), BUT ... it is bootable with the longer SCSI ID startup prompt (command/option/shift/delete/SCSI ID#), and if you install Apple CD-ROM 5.3.1 (from System 7.6) in place of your OS's standard CD/DVD driver/extension, it will work just fine as your primary/only CD drive. Just keep the Toast extension in another set for when you want to burn.