I had updated my Mac 3 days back from Elcapitan to Sierra. I had dualboot(Ubuntu + OSX) and it was working perfectly with default boot as Ubuntu and by pressing option key I had OSX booted. But after update I cannot boot Ubuntu. I have important files in Ubuntu, please help me. Thanks.
sudo efibootmgr -o 80,1
Last time, I seemed to use the liveUSB I used to install Lubuntu, choose try Lubuntu before installing, then open a terminal and run the command.