I backed up my 2012 MacBook Air before installing Mountain Lion. The backup was a bootable Lion clone. These clones have always been bootable with either a Thunderbolt connection or USB 3.0. After installing ML the Lion drives will no longer boot. Pressing Option at startup brings up the external drive in the choices to boot from, but selecting the external drives results in no boot! I cloned the ML drive and the clone will boot externally. So, does the ML installation trigger some kind of hardware switch that no longer allows for the Air to boot from an external Lion bootable disk? Anyone else experience this? Seems very strange.