If you’ve changed your boot preference, and can’t get the BeOS to boot on your new preferred volume, grab one of the BeOS installation CDs and boot from it rather than from your hard disk — you can do this by inserting both the boot floppy and the CD at the same time, or by using the launcher program from your „other“ operating system, and hit the left shift key (for ppc) or the Spacebar (x86) when the Be logo pops up.
Select the CD and agree to the license, and then type „Control+Alt+Shift+D“ when the installer comes up to launch Drive Setup. Mount the hard drive that you have your erroneous Boot preference on, write down where it mounted the drive, and close DriveSetup.
Next, type „Control+Alt+Shift+T“ to get a Terminal window. Be warned, though, that since the CD is now your /boot volume, your hard drive will be mounted at /Drivename rather than /boot — you should have written it down, but it’s pretty easy to find it. Just go to the TOP of your directory structure – „cd /“ works well – and look for the name of your drive.
Change directory down into your BeOS drive, and look in /Drivename/home/config/settings/ for a file called „Boot settings“. Delete it.
Reboot, and your box should be working fine again.