I don’t like Fusion Drives. They are unreliable.
There, now that I’ve got that out of the way, here’s how to fix a Fusion Drive if it flips out. Do note that fixing in the context of this article is not recovering the data from that. To recover the data from a broken Fusion Drive, you need to get this from your backups. Don’t have a backup? Well, once you fix the broken Fusion Drive, making a new backup will be your first priority.
It used to be, in versions of macOS prior to Mojave, that you would have to type in a whole heap of commands into the Terminal to create a new Core Storage volume, add the physical devices to it and then finally format. This involved copying and pasting the device UUIDs and other fun stuff like this.
Fortunately, with macOS Mojave and higher, there’s now a simple one-line command you can use in the Recovery OS to perform the same tasks.
And, that is it. This command will identify the SSD and the HDD devices, correctly determine which is the big and which is the fast part of the Fusion Drive and automatically stitch them together for you – the whole process takes just a few seconds.
Note again that this will erase all the data that may have previously been on your Fusion Drive – it does not repair a broken Fusion Drive letting you keep whatever data was on there. No, I don’t know how to repair a Fusion Drive and retain data.