Hello
I have exact same problem as yours. And I tried almost every possible way to get it solved, but unfortunatly it was not possible. The only thing I have'nt encountered with is that EFI problem.
And whenever I'm trying power+option key, and then choosing the external disc ( my samsung SSD connected to my Mac via USB), it boots but after a few seconds the prohibition sign appears, then I disconnect my SSD by pulling the cable and then immediatly connect it back, after few seconds the prohibition sign disappears and the boot loading continues from where it left before.
Any Help. plz?