I upgraded my MacPro5,1 to El Capitan and now when the machine boots, a globe flashes for half a minute before continuing the boot cycle. Eventually it boots from disk. I've tried selecting my boot disk as the Startup Disk, but this doesn't change anything. I then upgraded to Sierra, but this didn't change anything either. It appears the machine is trying to boot from the network, but I never enabled this myself. How do I turn this off so that it only looks for local hard disks to boot from?