Mid-2014 Macbook Pro i7 15" OS 10.6.8 boot fails I pulled my hard drive from a kaput i7 Macbook Pro because it had Snow Leopard (10.6.8) with Rosetta Stone. I need to run older G5 software. I put the drive in an external case, and hooked it up via USB to the newer Macbook Pro. I see the old drive on the screen when I boot normally under 10.11.5. However, when I boot up with the option key down, I get to select the 10.6.8 drive - but all I subsequently get is a gray screen with a small apple symbol. What is the problem please?
With few exceptions, a Mac must run the OS X version that it shipped with as a minimum version.  The minimum OS for your Mid-2014 MacBook Pro is at least 10.9.4, so you won't be able to run Snow Leopard on it.