In late July Apple released a SMC firmware update for late model MacBook Airs and Retina MacBook Pros, that adds support for Mountain Lion’s Power Nap feature.

This SMC firmware update will prevent these MacBooks from booting into Snow Leopard. This means that if the MacBook was originally running Snow Leopard, and was subsequently upgraded to Mountain Lion, then this firmware update was applied… it will no longer have the ability to boot from Snow Leopard on an external volume or disc (or downgrade the MacBook to Snow Leopard by reformatting the drive and starting from scratch).