I currently have two Mac Minis, one is the current gen, another is a 2009 model both loaded with OS X Lion. I am trying to boot the new Mac Mini with the hard drive pulled from the old one using a SATA to USB adapter. When booting, I pressed and held the Alt key. When the old hard drive appears as an external drive, I choose that drive to boot the Mac. That's when the prohibitory sign appears on the screen.