There is a limitation in Mac OS 9.x that will not recognize a system on a volume larger than 190GB.
Backup any data you do not want to lose. Reinitialize the drive and partition it. Be sure that there is at least one volume smaller than 190GB for Mac OS 9.x to be installed on. Once this is done, there will be no problem booting.
Some customers have experienced problems with the current version of the firmware in older G4 machines. Sonnet suggests downgrading the firmware to the Version 5.13, which can be found at http://www.firmtek.com/download/SeriTek1S2_513.zip