If you are looking to replace the existing factory fitted hard drive with an SSD and you have a 2 pin thermal sensor - like one out of an old iMac's optical drive unit (or a spare ordered online) you can easily use this and replace the factory fitted hard drive thermal sensor. Be very careful as this connector can be fragile but place the replacement thermal sensor on top of your SSD and secure down with sticky tape. Works a treat! The iMac's fan's do not spin at full speed and you don't need any software to control the fan speed. The iMac is truly fooled :-)

the optical replacement caddy is required in order to fit an SSD as the SATA connector which plugs into the optical drive is smaller and will not naturally fit your SSD!! I couldn't see this mentioned in this guide.

Also to confirm I ended up replacing the main hard drive with a SanDisk 256GB Extreme II drive and to prevent the constant fan noise problem if you fit a spare 2pin thermal sensor by unplugging the original hard drive sensor it does actually work! I managed to salvage a spare thermal sensor from an old iMac from the optical drive and plugged it in and it works a treat!!

Be very very careful when you pull out this plug as it is extremely fragile and also reattaching the LCD thermal sensor plug and very very gently ensure that the pins are actually going into the connector before you push the plug in. i damaged the top part of the plastic housing which then made it difficult to insert the plug into the connector. I can't believe how easy it was to damage this connector!! If you don't attach the LCD thermal sensor plug correctly the Mac fans will go crazy at full speed. It took a good 15 minutes to get the pins to finally go into the plug and they can bend easily.