Underneath the far right side of the motherboard, you'll see another slide connector - pull it to the right

I originally forgot to pull out the other end of the hard drive connector - to simplify matters, pull straight down from the motherboard using the pull tab

That's all that should be securing the motherboard - now to remove, slowly wiggle motherboard while pulling it right - the left-side accessory plugs need to clear the side of the case. Once they are far enough away, use your left hand to pull the left side of the motherboard up - eventually it should just freely pull off of the case.