ha!! Got it!!! The reason it wasn't working was that the usb stick (and all that I've tried) had a different partition format from the (old) usual. In summary:

Problem: EE not booting from usb.
Reason: the usb stick I was using had implicit Mac partition!
Solution: format usb stick again setting MS-DOS partition format.

Thanks all and I hope this downgrade works now.
(I have to say that being a scientist and not being able to solve this puzzle was driving me crazy!!!!)