It really is a fragile cable.
I was replacing my optical drive with an SSD. In the process I somehow broke this fragile cable. It wasn't immediately obvious as the drive showed up correctly on the first boot, but stopped responding shortly afterwards. I first suspected the drive, but when I swapped the HD and the SSD, the trouble stayed with the cable, so I looked at the comments on people who had done this repair before and saw other people had broken this cable as well. It seemed the obvious suspect.
I had already taken my mac apart a few times in trying to troubleshoot the problem, and my research that led me to suspect this cable, so I took care, replaced it, put it back together and everything was golden.
I couldn't see any obvious damage to the old cable, nor did I remember doing anything that would have harmed it, so treat it carefully. As the title says, it really is fragile.