Chosen Solution
My fusion drive shows as 2 drives and when I try to repair it through disk utility, I get the following error message: BTW I have installed MacOS on the HDD and left the SSD empty. It runs very slowly. Kevins-iMac:~ kevinheath$ diskutil resetFusion Internally-located hardware disk device known to the currently-running macOS: Rotational (disk1) Volumes exported by partitions or storage systems hosted on the above device: Flash Storage (disk2s1) iMac HD (disk3s1) Your computer must have exactly 1 solid-state and 1 rotational disk drive
Give this Apple T/N a try How to fix a split Fusion Drive