I'm using early 2011 i5 MacBook Pro. I upgraded to Sierra but the speakers and USB are not working and the system is very slow. Then I restarted my Mac with (cmd+alt/opt+R) to Recovery Mode. I cleaned my hard drive now I tried to reinstall Sierra but in the logs I saw it is downloading from the Internet. I stopped the installation and restarted my Mac. After the Apple logo it shows nothing but the ?.
Then I boot my USB drive to El Capitan but it is installing Sierra from the Internet… it takes days to download with my internet speed.
Best Answer
If you have an El Capitan USB installer, the fix is very simple. Make sure the laptop is off. Plug the computer in, then hold the "Option" key while the computer starts up. Keep holding it until you see an image of a drive on the center of the screen. Hit the "Enter" key to boot from the flash drive. Then the computer will walk you through the steps to install El Capitan.
Let me know if these directions did not work for you or if you need more explanation of these directions.