Failed to install OS X El Capitan from OS Lion on my Macbook and now I can't return to OS Lion
I have a MacBook Air Mid 2011, I tried installing an El Capitan update. During installation, it says, about 19 minutes remaining, then, "OS X COULD NOT BE INSTALLED IN YOUR COMPUTER. File Syste verify or repair failed. Quit the installer to restart your computer and try again. ", and every time I restart it, it just goes back to OS X installer.
One thing about my MacBook is, the WiFi doesn't work, so I cannot go to internet recovery. I also tried getting a TPLink Nano USB WiFi adapter, thinking it may detect wifi, but it doesn't (I think I need to install the drivers first, but to do that, I need a working OS)
PLEASE HELP ME
[Re-Titled by Moderator]