This step should really be done _before_ step 19 - this will give a lot more space to remove the power cable connector.
Look at step 20 below - you see one of these connectors on the side of the IO board, just under the fingers.
Actually, just sliding a spudger under the wires and edging upwards is the easiest way to disconnect this connector - and most others, too.
By the time you get to this step, the hard drive can be easily removed by simply sliding it out over the logic board. Then you can simply pull off the cable with your fingers. When re-assembling, again, attach the power cable connector first, with your fingers and slide the logic board all the way in. Then slide the hard drive in over the logic board.
Some optical drives have a tiny-bit different shape and as a result, the bracket won’t fit when reattaching. I ended up cutting a small piece off the plastic bracket with a knife to make it fit.