If you have the iMac sitting upright, there is a potential risk of dropping the screws into the iMac casing. Be cautious or lay it down flat.

The silver spots on the circuit boards. You may not want to tackle this alone. If you touch these areas, you can possible get a shock -- harming you and your computer.

I found it wasn't necessary to remove the speaker from the left side to remove the hard drive. You can remove the screws, and then slide it over to the left — giving more than enough room to access the hard drive.

Did some investigation into this. I found that OWC macsales.com has a complete wire harness kit that adds the plug-and-play cable with the In-line Digital Thermal Sensor to eliminate fan noise and maintains proper system fan control through the iMac's System Management Controller. No software hacks, or third party solutions needed.

http://eshop.macsales.com/item/OWC/DIYIM...