I had a problem with the HD. After a wile it was unrecognized. But the same HD was nicely seen if I booted from a USB drive (unmount the HD, put it in a external box and try...)
Therefore I thought it could not be the motherboard or the HD. It had to be the cable.
The repair shop wanted 60 euros only to see my machine. So i took my chance and ordered the part. After the substitution of the cable the mac runs smoothly, and is a easy job to do. Costed me 40 euros instead of 200 ;) and gave me quite some satisfaction. Be careful not to mix the screws, because is always better to put back the crew in her original place.
I ear that mine is a rather common problem to those who upgrade their HD drive themselves. I suspect that is not the cable or the contact that gets ruined, but something to do with the sudden motion sensor. I might be wrong off course.
Always try to do it yourself, following a good guide like the ifixit ones off course, and never underestimate electrostatic shock.