So, my SO decided he'd be smart and install macOS Sierra on his MB over El Capitan. Decided he doesn't want it after all and has tasked me with trying to downgrade him to El Capitan. He made no backup so this is daunting. I made a 2nd partition on his computer to try a workaround but kinda stuck there to install El Capitan on new partition via the internet recover while starting up but, that partition doesn't show up under startup discs... Now what? Guess I could move his files to new partition and try doing the recovery on main partition? I dunno. I'm out of ideas.