Replaced cable - saved my laptop
Just around a critical period, hard drive started failing, permissions errors abouneded, then the system wouldn't boot.
Thought it was a bad system restore I did a while back, or my SSD (have optibay-style setup). Restoring from image, then even time machine restore (to avoid filesystem corruption) didn't work. Then I isolated to primary HD slot. I spent literally a half-dozen hours trying to diagnose and repeatedly restoring and trying out different boot scenarios.
A google search later, I was buying the cable.
Easy-peasy. Instructions did not cover re-assembly, which is made more difficult when re-applying sticky portion of cable.
Suggestion for sticky part - make sure you lay it down and connect the wires first, then peel sticker and re-apply. unpeeling first can lead to frustration (esp. with such a delicate piece).
Always internet search for key symptoms - I never would have diagnosed without other ifixit reports and finding by googling.