We recently ran into a weird booting problem on a fairly new Mac Pro. The computer couldn’t boot if we do a warm restart. It boots alright if we turn the machine off and wait for a bit while. Resetting PRAM doesn’t work. Resetting PMU doesn’t work either. The machine also doesn’t boot to diagnostic mode and you cannot boot it up using a CD or DVD.
After talking to Apple Support for a few minutes, we finally figured out the problem is the iPod connected to the machine. Once we disconnected the iPod, the machine is back to normal. Note that the iPod itself is also working fine.