"First here is the IFIXIT guide to get to the SSD: Mac Mini Late 2014 SSD Replacement.
If you jump to Step 9 you'll see the SSD if you system has one. In your case it doesn't The SSD connector will be missing! Apple does not supply the cable on the HD model only units.
Here in the IFIXIT teardown you can see the connection point in Step 8: Mac Mini Late 2014 Teardown. OK, so you'll need to get the needed cable and a screw to hold the Apple custom SSD blade (which is simular to a M.2 SSD).
Sorry no, you can't use just any blade SSD or what ever you have in your drawer. As this is such a new system getting the needed cable will be difficult."
