Mac OS X: Computer Won't Start up After Resetting PRAM
If you have a RAID scheme set up, your computer may not start up if you reset parameter RAM (PRAM) when you restart.
Restart your computer while holding down the Option key to select your startup system.
If this doesn't work, restart your computer while holding down the Command, Option, Shift, and Delete keys.
Startup disk info is stored in PRAM, so if a Mac can't find it's startup disk try these methods, then try zapping pram if they fail. I think there's also a key combo that will select classic vs. OS X on dual boot startup.