I'm thinking (maybe) while typing here.
The find starup disk does a scan for valid OS's. You scan didn't find OS X. What does the scan look for when identifiying an OS? I do not know.
Can your read from the OS X disk when in OS 9?
It would be an interesting experiment to copy over OS 9 to the OS X disk. I think you need to copy over just the system folder. Does the scan find OS 9 on the OS X disk. [ When I do the system level scan, I find all sorts of OSs. I find OS 8 for some reason. ]
This is the option key startup manager. Sometimes if volumes don't appear in Startup Manager (what you get when you hold down the Option key at startup), you need to reset the Mac's PRAM, NVRAM, and Open Firmware. Shut down the Mac, then power it up, and before the screen lights up, quickly hold down the Command, Option, P, and R keys, until the Mac has chimed twice more after the powerup chime.
Then, before the screen lights up, hold down Command-Option-O-F until the Open Firmware screen appears. Then enter these lines, pressing Return after each one:
reset-nvram
set-defaults
reset-all
"The reset-all command should restart your Mac. If so, you have successfully reset the Open Firmware settings."
http://support.apple.com/kb/TS1812?viewlocale=en_US