Hi, I have a MAC PRO and sadly my old hard disk died. I have tried repeatedly to install OSX to a new hard disk and whilst the install is successful, when it reboots, it hangs. So I force the restart and nothing happens, it does not find the hard disk where the installation took place. I have tried holding down alt key, still no joy. All it finds is my DVD with the install disc for snow leopard! I have tried booting from a USB to install Mountain Lion. Still no joy. I have tried holding down command, option, P&R to try and force a reboot, all that happens is it plays the reboot sound and then cycles through it again and again and agin. If I remove all Mac disks and replace the Hard disk with a Windows 7 hard disk (previous install), it boots up after holding alt key and selecting so the machine is OK it just can't see the Mac disks. Any suggestions gratefully appreciated.