As it appears the 42.2 beta 2 iso fails the "check installation media" test
with:
this is not an opensuse Leap beta medium

Reproducible always. 
I tried with usb-key attached to PC directly and via usb-hub. 
Reporting because I did not find this as open bug although it is surely known
to the devs. 
Thanks to Viljo Mustonen who confirmed my doubt. 

It would be important to fix this issue for two reasons:
a) usb-keys get more and more important for install
b) if the install of the beta fails (as in my case) you are left with the doubt
to blame the medium, which, in this case, is a "false accusation".