Won't recognize the hard drive (flashing folder/question mark)

Started with Mid 2011 MacBook Pro 15" with video issues that are well documented. Had the solder re-flowed. That lasted for 2 days. Up to this point it run flawless for over a year (bought used) and is now out of Apple Care.

Did enough research to know I could replace the always faulty 2011 logic board with a 2012 logic board (non-retina Mac). Got all the tools, followed the directions on ifixit to a T. All seemed to go well (no issues with the swap at all). The only difference was the 2012 board did not have heat sinks on two chips but also had no screw bosses for them. The CPU and video heat sinks matched up perfectly.

Anyway, went to connect the video cable and found it had been damaged in another swap. (I could tell the machine had been apart before when I broke it down.) Dead stop, order another cable. That came and got it replaced, all is good.

Ready to power up. Booted to a flashing folder with a question mark in it. I have gone thru all the key sequences using option and commands keys. The drive in not even listed in the disk utils. Tried all the resets including removing the battery and RAM. Nothing works.

Pulled the drive and put it in a drive bay, used USB to connect and it booted, slow but it booted fine. All devices present and working correctly.

Ordered and replaced the HD cable. Same problem. The cable had a bad kink in it so I got another one (good this time) and it still has the same issue.

Now I am puzzled. The only thing left in my mind is the logic board. The only thing is it works perfect except for recognizing the HD and letting it boot internally.

HELP!