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Release the Airport and Bluetooth cables from the white plastic clip, mounted to the upper left edge of the Drive Carrier. Just push down on the clip and it will open.

Pay very close attention to and make notes of the wire routing through the following steps. It would be easy to inadvertently pinch wires between the drive carrier and the Upper Housing on reassembly.

Before proceeding, check once more that there is no strain on cables and connectors.

Remove the (2) 9mm Torx screws securing the Faraday cage (looks something like a cheese grater) to the Upper Housing, with a T10 Torxdriver.

Then remove the (4) 19mm Torx screws that mount the Drive Carrier to the Upper Housing with a T10 Torxdriver. 2 of the screws are located on the right side of the Drive Carrier near the center and 2 screws are located on left side, near the upper and lower corners.

Before you remove the drive carrier, take note of the wire routing, especially of the red, yellow and black drive power cables. Hold the Drive Carrier from both sides and pull it straight toward you. It is a tight fit and may require some wiggling to get it out.

The fan assembly of your iMac is now exposed. This is a great opportunity to blow the dust out of your machine!

Disconnect the Hard Drive's Data and Power Cables, Pulling the Data Cable connector by its white plastic tab (not the cable itself), and the Power Cable by its outside edges. These connectors can be very tight. Don't get discouraged. Patiently wiggle them back and forth so that they release evenly to avoid bending any pins.

Remove the white plastic protective film from the Hard Drive and save it for your new drive.

Remove the (4) 6mm Torx screws that hold the Hard drive to the Drive Carrier with a T10 Torxdriver. there are (2) screws located on each side.

Your Hard Drive is free!

Important information for reassembling your iMac. You must clean and apply thermal paste to the mating surfaces of the CPU Heat Pipe and The Upper Housing, indicated by the yellow squares in the 2 photographs. Follow iFixit's excellent Thermal Paste Guide Applying Thermal Paste.