iFixit

Disconnect the hard drive thermal sensor from the top edge of the logic board.

Pull the connector parallel to the face of the logic board.

Remove the two 9.2 mm T10 Torx screws securing the hard drive bracket to the rear case.

Lift the hard drive from its left edge enough to clear the inverter and pull it toward the left edge of your iMac to unseat the mounting pins from the chassis.

During installation, do not push the rubber grommets through the chassis with the hard drive mounting pins as retrieving them will require removing the logic board.

Pull both the SATA power and data cable connectors away from the hard drive.

Be careful when de-routing the hard drive thermal sensor cable from under the heat sink framework.

Remove the two 12 mm shouldered T8 Torx screws holding the hard drive bracket to the connector side of the hard drive.

After removing these two screws, the bracket will fall away from the hard drive.

Remove the two 9.2 mm T8 Torx pins from the other side of the hard drive.

Use the flat end of a spudger to pry the thermal sensor board off the adhesive securing it to the hard drive.

If you're replacing your hard drive, transfer this sensor to the same position on the new drive.

Users have reported that without the sensor connected, the fans will run consistently at maximum speed.