Will no longer boot past white screen

@jurgenkoppen - I don’t think you need to get into the logic board to fix this. I think you have the classic case of bad HD SATA cable! If I understand correctly you can boot the system up, just not with the internal drive (which ever you put in)

So you’ll need a new cable MacBook Pro 15" Unibody (Mid 2012) Hard Drive Cable and here’s the guide to put it in MacBook Pro 15" Unibody Mid 2012 Hard Drive/IR Sensor Cable Replacement. Don’t get the wrong cable! There are a few versions and I only recommend the newer version.

What I also strongly recommend is to place a strip of electricians tape on the uppercase where the cable rests Your Hard Drive Cable Is A Ticking Time Bomb

A second issue is the mid plane clips tend to break! If one is good move it over to the optical drive side.

If you don’t have a good one place strips of foam tape as shown here to protect the cable from damage so the bottom cover can’t hit it.

The last issue is not damaging the cable when you install it, I often find people damage the cable by creasing it where the cable needs to bend around a corner. Don’t do it! The thin foil wires are easily damaged! I use an old BIC pen ink straw as a forming break so I can bend the cable in a nice arc so the cable has no sharp bends.

You will want to reformat the drive as its likely you have some corruptions and then re-install the OS. That should do it!