Hi All, Just wanted to share my experience of upgrading the 2012 Mac mini - specifically roping in the out of control kernel_task post Fusion Drive swap. As Apple is refusing to upgrade their Macs, or at least not in ways which are always that ideal, squeezing extra life out of our current machines is essential. I was running our of space on my custom 1.25TB FD in my mini, so I purchased some Black Friday deals and got a 500GB SSD and 2TB HDD to double my space in a new Fusion Drive arrangement. Swap made, cloned 2.5TB FD installed, all was well - except things heated up as my CPU went haywire. TL;DR booting into recovery mode and running first aid in Disk Utility solved the issue. It seems that cloning across the old data was the issue as the Fusion Drives were disparate in size which made the CPU spin up massively. Hope this can help someone else in the future.