I had the exact same experience as tom. I installed a brand new 2TB Samsung 850 EVO and it was not recognized by the Mac Mini. I booted from a portable USB drive with a bootable copy of macOS High Sierra but the 850 EVO SSD I had just installed was not shown as an option. I tore down the Mac Mini again, formatted the 850 EVO, rebuilt the Mac Mini, and booted off the same bootable High Sierra USB drive, and sure enough the 850 EVO was recognized.

This is a very fiddly connector. Afajner’s tip is recommended. Make sure the connector is in securely. It will not wiggle or make a ticking sound when pressed when it is seated correctly.

When re-seating the PSU, be sure to align the little metal tab that points toward of the front of the Mac Mini so that it mates with the receiving socket at the front of the case. If not, the light at the front of the case that glows white when the Mac Mini is in various power states will not illuminate.