I encountered more and more performance issues with our iMac. I could have exchange the whole HDD. However, I choose for a Fusion Drive by installing a SSD into my old iMac.
The repair went flawless. I had some experience with taking my iMac apart. I didn't remove the screen; I just lifted it and secured it in 45 degree position. This gave me more then enough space to remove the DVD drive and to install the PATA Optical Bay SATA Hard Drive Enclosure. After installation of the SSD I ran the commands to establish my personal Fusion Drive. And I succeeded!!! My iMac is now faster than my 2009 MacBook Pro. I see a next project coming up ;-)
When in the process of combining the HDD and the SSD the command is create a Fusion Drive by entering the following 'diskutil coreStorage create <yournameforthenewvolume> <drive1> <drive2>. Drive 1 being your SSD and drive 2 being your HDD. I was instructed to use the main disk level (disk0 / disk1) to succeed. However, because my HDD also contained my recovery info, disk1 could not be de-activated. My solution was then to use disk1s1 which is the non-recovery partition on my HDD. This worked!!