SSD Upgrade with HDD Thermal Sensor

There is a second option instead of replacing your hard drive, just add the SSD using the dual drive adapter as outlined here: Installazione doppio hard drive iMac Intel EMC 2308 da 21,5" If your HDD is still working this would be a great way to gain the speed of the SSD making it the boot drive and apps then leveraging the HDD for your data and other content.

Sadly, the way the systems thermal management works requires the sensor to be present to measure the drives heat for SMC. As the sensor is within the hard drive we need to replace it with what SMC expects. The OWC in-line sensor OWC In-line Digital Thermal Sensor for iMac Late 2009 - Mid 2010 Hard Drive Upgrade the 2011 unit is different unit!

Some people have tried using a spare optical drives sensor, it's not as good as the OWC sensor. When I was swapping out drives in this series I just found the OWC sensor was just easier.