External Boot = No Entry Sign

[scrambler01]

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.

 

[chrfr]

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.

Yes, the newest computers will a special version of 10.10.3, or 10.10.4 or later. If any of your Macs are running any version of Yosemite earlier than 10.10.5, it's worth updating them.