Mac mini Problem - Won't boot after upgrading to High Sierra (Bless) I bought a used Mac mini from a reliable source. He factory reset it, so we set about upgrading to the highest we can, High Sierra. I created a bootable USB on my iMac using the High Sierra installer. Inserted it, booted from it and installed the OS. 

All looked good, and we got into the OS on first load. Upon restarting, it says “MacOS could not be installed on your computer. An error occurred while loading the installer resources”
"Quit the installer to restart your computer and start again". Weird, because the installation has finished and the USB drive is no longer in there. 

It also mentioned something about bless but I can't remember what. 

This machine has an SSD, I THINK when it arrived there were two partitions but only one now. I have no idea about SSDs, never used one before. Does it need a separate partition for booting or something? 

Totally lost, would be very grateful for some help.
If you startup with the USB drive again, try to use the Disk Utility (available pre-install) to set the Startup/Internal Drive ("blessing" it).