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Tl;dr learn from my mistake: Don't mess with iCloud, unless you've really lost your laptop. Yesterday I tried to activate Lock Mode to test it(!) and the OS rebooted back into Mac OS without any issue, unlike my iMac which rebooted into the "This Mac has been locked" screen. So I thought nothing of it. Until today where I was going into the installer to access Terminal and the lock screen popped up, though the PIN I set didn't work and iCloud didn't recognize the laptop as locked. Oops. Things I tried: - Wiping the OS = Still locked. - Removing the battery and reseating the CMOS battery = Still locked. After Googling for a while I found that you need to clear the NVRAM to remove the iCloud lock key. When you reach the Clover menu of your OS X Install USB, add the arguments -s and -v to the boot arguments and select the Install OS X <Version> option on the first menu. When you get to the bash prompt run nvram -c (or a few times if it makes you feel better after making the same mistake) and reboot. Your Mac should no longer be locked! Since I decided to re-install afterwards, I can't guarantee it won't re-lock if you don't re-install and keep using your existing installation, but at least you'll be able to re-use the laptop for OS X again. Learn from my stupidity. Don't play with iCloud.