I am trying to install Snow Leopard in a partition. (The partition has nothing on/in it) on my mini's internal HD. The OS is Yosemite. I get this message; If I double click the installer. If I drag the installer to the Snow Leopard partition the installer goes there but when I click on it I get the same message? Also....If I install SL on my external HD (that has Time Machine on it) will that mess with Time Machine? Thanks.