Be careful when replacing the HD/SSD in this step. If you are not careful, you can mess up the rubber shock absorbers by misaligning the posts. If that happens, you will have a hard time getting the screws to line up when putting the perforated grille back on (wifi antenna grill).
Be careful to LIFT UP on the fan connector cable. Using a spudger tool helps here. I mistakenly thought that the connector would slide off the logic board and tried to remove the connector this way. I ended up lifting the connector off of the logic board, thus ruining my logic board and requiring a $350 logic board replacement at the genius bar. The second time I did a HD/SSD replacement, everything worked fine.
Be careful to not break the nylon pins during removal or insertion. The strength of the spring can break the barbed end off the pin.
Also, be sure to orient the heat sink in the right direction when putting it back. The fins should allow air to flow towards the openings at the back of the Mini (where the ports are).