Yesterday, my MacPro running the latest build of snow leopard froze and I had to force a reboot. After the reboot everything started acting up so I ran a permission repair but that didn't solve the problem. I then ran Onyx which told me there was a problem with the startup volume and it had to be repaired. I have both my Snow Leopard install disc and also a bootable backup on an external hard drive. I tried rebooting from the bootable backup and whileI was able to select it by holding the option key, it wouldn't boot. I then tried to boot from the install disc while holding the c key, and the computer would either go into a cycle of trying to boot and then restarting itself, or gives me the error message that "the computer has to be shut down by holding the power key." I then tried to boot in safe mode by holding the shift key and the same thing happened it wouldn't boot. Note that I am able to restart into my regular user mode, but the system is clearly malfunctioning and needs to be repaired. Any ideas how I can repair the startup volume?