I'm trying to do a clean install of Snow Leopard on my friend's MacBook Pro. He's currently running Lion. I have tried to boot from both a 10.6 CD and a USB image, but it just hangs on the grey apple logo with no spinning wheel. I also tried to run Disk Warrior from a CD, but it wouldn't boot that either. The CDs and USB work perfectly on my MacBook Pro, so that's not the problem. I tried resetting the NVRAM and PRAM but that didn't help either. Any idea what could prevent a MacBook Pro from being able to boot from any CD or USB image? Thanks for the help.