2009 Mac Mini HDD as an External Drive for a 2011 Mac Mini HDD
Intel Macs can boot to a USB volume, but in this case it will not work. The reason is because the new Mid 2011 macs ship with 10.7, as you mentioned. Macs ship with a minimum supported OS, and will not boot or allow installation of any OS prior to that. Therefore, a Mac that shipped with 10.7 will not boot to a 10.6 install. You'll probably be able to see in when holding option at startup, but if you select it it will likely show an Apple logo and hang there.