For fun, I've bought a USB thumbdrive from a reputable brand, and was able to build an APFS fusion drive (with the internal SSD as the main, and the USB thumbdrive as the secondary) on my 2018 Mac Mini, which came with Mojave, as follows:
- Reboot into Mojave's recovery
- In Startup Security Utility, choose "Medium Security" and choose "Allow booting from external or removable media"
- In Terminal, list all disks
$ diskutil list | head -40
- Then delete the APFS volume:
$ diskutil apfs deleteContainer disk0s2
- Then create a Core Storage Fusion Drive:
$ diskutil cs create "Fusion LVG" disk0 disk4 $ diskutil cs createVolume <identifier> jhfs+ "Fusion" 100%
- Exit Terminal and go to Disk Utility. Format the drive as APFS.
- Reboot into internet recovery mode with Opt-Cmd-R and install Catalina.
The problem is as follows. The installation finishes correctly, and leaves you with a running system. However data gets corrupted when the fusion drive contains too much data to solely exist on the internal drive. I've checked this by copying one file multiple times and then checking the MD5 sum.
Why have I been able to create a fusion drive that actually corrupts data? Why did it go wrong?