Macbook pro was booting to home a flashing file folder, trying to recover showed now hard drive. Ran hardware diagnostics and there were no issues found, and tried replacing the hard drive with a new one. No dice. Last ditch effort before going to apple and getting hosed down for a new logic board was the cable. The internet and iFixit users had a lot of reports of this resolving their issue. I was skeptical because I don't really understand how a fixed cable would go bad, but c'est la vie.
Process was straight forward. iFixit recommends pulling the battery and a bunch of other work that I didn't find necessary. You can remove the upper half of the top hard drive bracket and the bottom hard drive bracket and that should be enough to get the old cable out and the new one in. My cable was a little bit long, I'm worried about that causing problems over the long term, but it fit back in the case so if it can limp this 5 year old laptop a long for $50 and another year or two then that's fine. I went ahead and installed the new hard drive instead of the original one with a hybrid drive that has 1tb (previous was like 320gb or something tiny). Apple Migration Assistant did the rest using a usb to sata cable.
If you are having a similar problem I would recommend testing the old harddrive using a usb to sata cable in order to make sure it's not the drive before trying to replace it. Use the free tools that Apple provides before just jumping into a $50 cable. Isolate all of the other likely suspects first. Our repair ended up being $130 total because of the new drive, but the new drive wasn't strictly necessary and I wish I would have thought to check that down first.