Hi all, I am running HS 10.13.4 and I have been trying to install Mountain Lion on en external drive so I can use dvd studio pro. So far, all I have tried has failed. Whenever I restart form the installer USB stick all I get is a message that the mac was not not shut down properly and the mac boot again with High Sierra. Any ideas? My mac is a 12-core 2010 Mac Pro. Thanks!