I have a 2012 Mac Mini with a working High Sierra install on a SSD (APFS Volume) in an USB3 external enclosure mounted on /dev/disk2s1 according to Disk Utility
After I moved that same SSD to the internal SATA HD slot, it no longer appears as a boot drive. Disk Utility would still recognize the SSD on /dev/disk3s1 as a SATA drive. Also tried resetting the NVRAM but that did not solve the problem.
I moved the SSD back to USB3 and everything still works fine.
Update
I backed up my data with the SSD connected by USB3 and then tried a time machine recovery to the SSD installed as SATA. Also tried a fresh install to the SSD after erasing the partition. After both efforts, the SSD still will not appear as a boot drive when I hold the option key on startup.
any ideas?
Best Answer
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.