When it came out on Mac, I had a Mac Mini with a tiny internal drive (120 gigs). I tried to make an APFS Fusion drive where I fused the internal plus a fast USB

When it came out on Mac, I had a Mac Mini with a tiny internal drive (120 gigs). I tried to make an APFS Fusion drive where I fused the internal plus a fast USB-attached thumbdrive together. That seemed to work but would then fail upon reboot, because macOS would renumber devices and then couldn't find the drive. Under Linux, you can simply assign /dev/sdX to a particular drive, just create an udev rule. Under macOS, that's impossible to my knowledge.