Questions on upgrading drives (HDD Vs SSD) and how.

@jvemo - Jon We need to start over here as this system is different than the 2011 model!

So lets start with the systems SATA ports in this series they are SATA II (3.0 Gb/s) so the drives you use must support this slower I/O speed.

Here is the IFIXIT guide to replace the HD: iMac Intel 21.5" EMC 2308 Hard Drive Replacement. Similar to the 2011 model this system leverages the HD's internal thermal sensor. But unlike the 2011 model this model uses spare pins on a jumper block to gain access to the sensor. But! There are different connectors depending on the drive suppler Apple used for your given system (three different sensor cables!) In addition the header on the drive has more free pins for the cable so you could mix up the connection! This could explain why your fan is racing as the cable is not plugged into the correct pins! You can see in the guide on Step10 what I'm talking about here pictorially.

As far as adding a SSD, This model only has two SATA ports unlike the 2011 which has three. So you'll need to rethink things a bit as you'll need to give up your optical drive for the SSD. Here's the IFIXIT guide: Installing iMac Intel 21.5" EMC 2308 Dual Drive.

So your options are either do a dual drive as described above or make the jump to a 3.5" hybrid SSHD drive replacing your current HD. Which I think is the better direction.

Here is what you'll need:

• FireCuda the 1 TB is most likely enough for you if not go with the 2TB. But you'll need to have at least 8 GB of RAM.
• OWC - In-line Digital Thermal Sensor for iMac 2009-2010 Hard Drive Upgrade. We need a new sensor (external now) to replace the now missing one.