The MacBook was coming up with a 'white screen'. Eventually it changed to an icon of a file-folder with a flashing question mark. This is the indication that the computer can't find the hard drive. I had installed a newer larger hard drive (also from iFixit) earlier this year, so I was worried that the drive was faulty. However, I opened the bottom of the computer and ran the following tests:
1. Pulled the drive, mounted it in an external USB enclosure, and booted the computer from that. It worked.
2. Installed the older drive in the computer and booted. Got the white screen, etc.
3. Disconnected the hard drive cable and cleaned the ends with electronic cleaner. Reconnected and rebooted several times. Sometimes white screen, sometimes successful boot.
4. Physically pushed on the cable gently while booting. Got successful boot.
Now the cable was suspect. Other people on iFixit reported having bad cables, so I decided to try a replacement.
The repair was successful. With a new cable in place, the computer boots correctly every time.
It is tedious to get the old cable out, due to the fact that the cable is glued to the computer in two places. (On the underside of the palm rest, and at the bracket at the front of the computer.) I had to peel it off very carefully.
1. Use a good magnifier to inspect the connection to the bracket at the front of the computer. There is a narrow cable that goes from the hard drive cable to the hard drive indicator light. There is a little clasp that secures the narrow cable. That clasp must be lifted to release the narrow cable. Be sure to note how the narrow cable is inserted, because you need to re-insert that narrow cable into the new hard-drive cable.
2. I did use the supplied peel-and-stick tape to secure the hard drive cable to the front bracket (after I had re-inserted and locked the narrow cable to it).
3. I did NOT use the peel-and-stick tape to secure the hard drive cable to the underside of the palm rest. I decided it would be difficult to get the cable positioned properly. Instead I put a length of electrical tape on top of the hard drive cable. Kapton tape would have worked well, as an alternative. I did not notice any additional mechanical pressure on the hard drive after as I installed the hard drive.