Fusion Drives
[TODO]
Erasing
Catalina
Boot into testing drive.
Use Disk Utility to break Fusion, then secure erase each drive separately.
Use DeployStudio to create a new Fusion Drive and deploy.
Repairing
Full explanation here: https://support.apple.com/en-gb/HT207584
High Sierra and earlier
Boot into the newest Internet Recovery with Option + Command(⌘) + R (or: Command(⌘) + R) and open terminal.
diskutil cs create Macintosh\ HD disk0 disk1
diskutil cs list
Use the Logical Volume Group GUID in the following command
diskutil cs createVolume logicalvolumegroup jhfs+ Macintosh\ HD 100%
Mojave and later
Boot into the newest Internet Recovery with Option + Command(⌘) + R (or: Command(⌘) + R) and open terminal.
diskutil resetFusion