APFS Fusion Drive with external USB on 2018 Mac Mini 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 &lt;identifier&gt; 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?
This is a bad idea for lots of reasons. You're lucky that the Mac can boot from a Fusion drive that's half T2 and half external USB.

I suspect the T2 disk controller is at the root of the problem. Possibly because of its built-in encryption. Perhaps the Fusion magic moves encrypted blocks off the SSD without decrypting them?

Even if it did work, using a USB thumb drive is going to massively compromise the speed of the  fast internal portion. 

I don't think that 'expanding' the internal storage into a bigger logical volume, either with Fusion or RAID is going to be workable.