I needed to remove the PSO completely to reach the baseboard drive cable end ZIF. To do that disconnect the power and data (I assume) cables and then pull the unit straight up. To longish prongs are heading straight up into it from the baseboard and it is a bit tight, so gently pull until it's clear.
The top would hardly budge on the left back corner and was stuck on the right back corner. There is a plastic hook that needs to be unfastened to lift the top off. It is located in a hole on the top back corner on the right hand side, so the corner straight across the on off button. Just gently push the plastic hook a bit front the rear of the machine with a spudger. It gives easily and the top will be free to lift off after that.