Sorry if this is all been talked about here before but: I have been running Yosemite on and external Thunderbolt Lacie SSD and my mac mini for a while. Today I bought a new MBP with Yosemite installed. I wanted to boot off the same external drive. I could see it and select it but then instead of booting all I got was the 'no entry' sign- the circle with a diagonal line through it. I've try everything I can think of, including cloning the external drive to a traditional drive with USB connection. Same problem. PLEASE PLEASE, can anybody shed some light on this issue? Thanks in advance. -EDIT- OK, I think I've solved it. The slightly older version of Yosemite maybe doesn't run on the latest MBP chip/hardware. If you don't hear from me again it's because a simple OS update fixed the problem.