I had the same problem, but this fixed it:
Go into startup disk and wait until it's done checking all the disks
Go back into classic, and (it worked for me...) the disk should be selectable
Whenever I startup classic with the extensions on it quits right after it's done booting up. Even in the OS9.1 base extension set it does this. I'm running a B&W G3 350mHZ 1gb ram. Anyone know anything about this? thanks