Chosen Solution
My MacBook Pro Mid 2012 15inch Unibody is getting stuck on boot up. The bar makes it about half way and sits there. The motherboard is 820-3330. I have changed the HDD out with a known working one to no avail. The machine will not boot off of a USB HDD. The machine will boot the Apple Hardware Test from a USB but will not find an issue.
You have a classic case of worn HD SATA cable! You’ll need to get a new cable MacBook Pro 15" Unibody (Mid 2012) Hard Drive Cable and you want to follow this IFIXIT guide MacBook Pro 15" Unibody Mid 2012 Hard Drive/IR Sensor Cable Replacement. You also want to apply a strip of electricians tape on the uppercase where the cable crosses over. In addition you don’t want to over fold the cable! You want smooth arcs not sharp bends. The thin foil wires are fragile! Which you don’t want to damage. I would recommend you reformat the drive (after backing it up) to make sure any errors are cleaned up.
