Macbook Pro SATA Cable replacement
Hard drive would not boot while connected to the internal SATA cable but would boot correctly when placed into an external USB carrier.
The repair process was very easy. Remove the laptop bottom cover, unscrew and remove the hard drive hold down brackets, unplug the hard drive from the SATA cable then remove the two hold down screws for the SATA cable and unplug the cable from the motherboard. Install the iFixit replacement SATA cable in the opposite order.
Go to iFixit for advice and parts for Apple product repair. They'll save you allot of time and money.