DriveDX was originally installed on the 500GB internal spinner running ElCapitan.
Then I decided to upgrade to Mojave, but I could use more internal storage and I also wanted to switch to an SSD for better performance with APFS so I bought a 1 TB Crucial SSD.
Pressed for time and a bit cautious with my main Mac, I decided to run Mojave on the SSD in an external USB 3.0 enclosure and keep the internal ElCapitan spinner as a back up to fall back on if I didn't like Mojave.
Plan was to run like that for a ouple of weeks and if no issues install the SSD in the Mini to replace the spinner.
Well, it's been a few months now and I still haven't found the time to do the switch - mostly because other items mone up in the priority list and running Mojave as the boot drive on an external USB 3.0 enclosure works just fine.
DriveDX was migrated from ElCapitan on the spinner to Mojave on the SSD along with everything else via a clean Mojave install and Migration Assistant.
Now when I boot into Mojave, I can launch DriveDX but only the spinner shows up, so it seems the SSD is considered "external" even though it's the boot drive.
I will probably just wait until I move the SSD into the Mini.