Mac Mini 2012: Cant Boot after moving SSD from external USB3 to internal SATA
SOLVED

I ended up swapping SATA positions on the SSD and HDD so that the SSD is in position 2 (nearest the bottom of the Mac Mini).  After that change, plus a Time Machine recovery, the SSD shows up as a boot drive and everything seems to work fine.

I'm not sure why this worked, but it did.