I am posting here because so many people boot there mac mini's from external SSD drives and may have had this issue. A while back my parents mini started having problems with their computer starting up. Even when I started in recovery mode, I could not get the disk tool to see the internal drive. I ended up getting an external USB SSD drive and installing Sierra and all was well. Things were much faster, nobody really even noticed. Then one day I got a call that the computer was acting funning, and low and behold, I discovered the old internal HD had came back to life and was bypassing the external SSD drive. I ended up reformatting the internal HD and just left it alone. I didn't think I could trust it anymore and didn't want it to accidentally boot up over the external SSD drive. It must have worked for a while because I managed to install High Sierra w/o any problems in August. Yesterday I tried to install Mojave and I get an error that there is no "firmware partition". The internal HD must be dead again, no matter how I boot up or try to use disk tool the internal HD is not seen. I have another mini and think i can trick Mojave to install itself onto the external SSD drive by plugging it into another mini, but I will be left with the same problem in the future. I am trying to avoid potential damage by opening my parents mini and installing the ssd drive. I have already broke a status light when I tried it once before. Has anyone else dealt with this issue of a running an external SSD drive, after the internal HD has died or can no long be seen?