Thanks, @danj. Same issue (after changing the logic board and keeping my original SSD - where I installed Mojave. SSD is not visible) but when I am using the Mojave bootable USB thumb, all possible drives (all external USB thumbs, in one I have the current OS Sierra 10.12.6 ... the last possible updates I was able to perform) are light grey and they can not be used with the following message "You can not install on the volume because the computer does not have a firmware partition". I tried all your suggestions but none is working. Any more suggestions? Thanks in advance