In my case, it just so happened that my iMac needed to go in for service (the video card, SuperDrive and display went bad) and while I had it in there, I asked the technicians to install the SSD behind the optical drive for me. Here are a couple of videos that show you how to do this: You can choose to do the installation yourself if you are so inclined, but realize that you may void the warranty on the original hard drive if something goes wrong, and this is according to Apple Tech Support, with whom I checked prior to ordering the kit. The kit includes the suction cups used to remove the screen (held into place with magnets) and a screwdriver set. You will also need a special SATA wire, which is sold as a kit from both OWC and iFixit. The hardware installation is somewhat involved, as you will need to not only open the iMac but also remove most of the connections and also unseat the motherboard so you can get at the SATA III connector on its back. It’s a SATA III (6 Gbps) drive and when I look in System Info, my iMac sees it as such and is able to communicate with it at 6 Gbps, which is really nice. I purchased a 128GB Vertex4 SSD from OCZ. I bought it with a 1TB hard drive, which I recently considered upgrading to a 3TB hard drive but decided against, given the fan control issues with the temperature sensor and the special connector used on the factory drive. My machine is a 27″ mid-2011 iMac with a 3.4 GHz processor and 16GB of RAM. This results in a big performance increase without having to fiddle with bash commands too much. Fusion Drive works automatically once you set it up, moving the files that are used more often onto the SSD and keeping the ones that are accessed less often on the hard drive. Some geekier Mac users will likely prefer to install an SSD and manually separate the system and app files from the user files which take up the most space, which is something that gives them more control over what works faster and what doesn’t, but that’s a more involved process. I like Fusion Drive because it’s simple and automated, like Time Machine. I’m not sure how this method will work with Macs older than 2011, but I know for sure that it works on mid-2011 iMacs, and quite possibly on other Macs made since then. I have just completed this process for my iMac and I thought it would help you if I detailed it here.

Yes, you can enable Fusion Drive on older Macs.

Hard drive for imac 2011