Q: Can't boot on my external hard drive on another Mac (stop sign)

Hi,

My system is placed on an external SSD connected to my iMac.

I've just bought a brand new Mac Mini and I wish to boot it using my SSD.

When I pick it up in the startup alternatives list (maintening the "Alt" key pressed during the boot), I just get a gray screen with a stop sign at the middle. Then the Mini shuts down.

Isn't it possible at all to use the same boot disk on my iMac and on my Mini ? (I don't intend to use it constantly on both, I was just hoping to migrate flawlessly using the same System disk).

Note :

- when I plug it back to my iMac, the iMac boots up normally

- if I boot my Mac Mini using the internal hard drive, I can access to all the content of my SSD. I performed a full disk check on it using the Disk Utility and everything's fine. I just can't boot with it.

- of course, both of the systems (iMac/SSD and Mac Mini internal HDD) are up-to-date (10.8.2)

Thanks for your help.