Surprisingly, it was from something as low level as creating a new user in the Settings panel of OS X. I was sitting with him.
Linux boards say they can't fix it, so I have been looking for places that know about OS X's command line.
Do you know any other places where people dig deep into OS X?
It used to boot straight to Ubuntu.
I used the alt/option menu on startup to get to OS X.
Ubuntu didn't show up on that menu.
Now it boots straight to OS X.
A Mac tech added a user in OS X right before it stopped working.
How it worked before
I installed Ubuntu and OS X like...
I need to change the start up disk to be a different disk instead of my OS X disk.
OS X cannot see the disk I want it to use.
I cannot change the start up disk back in OS X's settings.
How do I use the command line in OS X to fix it?
2009 Macbook Pro
OS X El Capitan