Apple has used a Fusion Drive setup in its iMacs for years now. A Fusion Drive is a combination spinning hard drive and SSD drive combined to make one main drive. The spinning hard drive is used for storage and the SSD is supposed to run the system and Apps. Even though iMacs are moving toward straight SSD storage, there are still many Fusion drives out there. If you are running a Fusion drive iMac and it is failing, then read this article which describes different things you can do to fix it and move on.