I tried to install Mac OS X El Capitan and it failed in ~70% progress after reboot and now I even can't start recovery mode (pressing Cmd+R just tries to continue installation process and fails again and again).
Assuming I can get a backup of my user files over Target Disk Mode and then hand migrate them, what steps do I need to perform to wipe and reinstall Yosemite on my Mac?
Best Answer
If you have another Mac, you should make a bootable Yosemite drive. To do that, you need an 8GB USB drive and the Yosemite installer downloaded. Here are the instructions, copied from the linked article:
When that's done, plug that USB drive into the El Capitan Mac, hold down Option and turn the Mac on, and select
Install OS X Yosemite
from the list of startup drives. That should bring you into something that's identical to the Recovery HD for all intents and purposes (the only difference being that you don't need to redownload the OS X installer; it's stored locally). From there you should be able to wipe the hard drive and reinstall Yosemite. Good luck!