Messy Fix Help

[MacAlien]

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.

 

[AndyK]

A simple google of 'downgrade from Sierra to El Capitan' will give you two things.

1. OS X / MacOS is not downgradable.
2. A large amount of guides how you can re install El Capitan on a new partition and remove the Sierra partition. This for example.

If he made no backup and you've not made a backup before you embarked on your attempt, prepare yourself for dataloss. You might be able to boot back into the Sierra install and pull them off onto a USB or something. This might be useful.

 

[KALLT]

The safest way would be to clone the Sierra partition onto an external drive and then perform the El Capitan install on the main disk.

 

[Weaselboy]

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.

Run the two commands below in Terminal then post the output of each command here for us to take a look at your partition layout. Also, tell us (from that output) what is on each partition. Then we can walk you through what to do next.

diskutil list

diskutil cs list