Had this crazy experience that after having replaced the logic board my laptop did not want to boot from the SSD - just got the gray screen with the blinking folder icon. The same ssd booted happily on another laptop. Stuffed in an old drive with El Captain, no problem booting from that drive, so computer was working. Then spend some hours upgrading the El Captain to High Sierra, ran all the updates. Attached the original ssd again and ask to restart the laptop, it worked.

Not sure what this indicates, except that I am very happy I had that old drive lying around!