iMac 3TB fusion drive layout

No, it should just be one big partition. OSX will automatically find out which things to store on the SSD and which to store on the HDD.

I would reformat it. Start up in recovery mode, and go to Disk utility and format the hard drive to one partition and reinstall OSX.

 

Although not delivered from Apple configured that way, it is a popular user option for Fusion drives when you want drive space not managed by Fusion for large "static" files such as movies, video, and music files. Some users might also install Windows (BootCamp) on the free partition outside of Fusion management.

If you wish to return it to a single large Fusion drive without reformatting and reloading everything, the "BootCamp Assistant" under your Applications/Utility folder should be able to "undo" the partitioning.

 

Which is daft - either these files are not used much, then they stay away from the SSD drive, or they are used a lot, in which case they move to the SSD _as they should_. And if an app isn't used, then you don't want it on the SSD.

 

Large files which won't benefit from SSD speeds and aren't being modified once saved (movies, music, etc.) can be isolated to a separate drive or partition which will keep them from being moved to the SSD by the Fusion algorithms, even if they are accessed every day. Much better for your SSD and overall system performance.