To change your startup disk to OS 9, type this in a terminal window:
sudo /usr/sbin/bless/ -folder9 '/Volumes/Mac OS 9/System Folder' -setOFNOTE: Replace "Mac OS 9" with the name of your Mac OS 9 partition.
To change your startup disk to OS X:
sudo /usr/sbin/bless/ -folder '/System/Library/CoreServices' -setOFI put this into an AppleScript to make it even easier by calling a "do shell script" command. This sure has saved me a lot of time getting around the "Startup Disk Pref Pain". For more information regarding "bless" launch the "Terminal" and type "man bless" (without the quotes) for the "bless" manual pages.
[Editor's Warning: If you're relatively new to the command line, you may want to make sure you understand what you're doing before trying this hint. "man bless" will give you an overview of instructions as a starting point.]

