I have a 2014 Mac Mini.
I've seen a number of these issues but haven't found anything that could help.
When trying to upgrade to Mojave, I get:
you may not install to this volume because the computer is missing a firmware partition
I boot and run off a SSD hooked up to a USB. Hooking the drive up internally doesn't work. I brought it in to get looked at and was told the computer is shot.
When I brought it home I figured I would try hooking up an external drive and it booted and works fine.
Anyway, since I do iOS development, it is critical that I be able to update to the latest version.
Any suggestions on how to get around not having the drive internal?
Is there a way to add a firmware partition?