So someone at work trashed his home Mac's Hard Disk and brought it in to see if we could get it to work. I opened up my Macbook, pulled out my HD and put his in, and we just got random clicks and no booting happening. His disk was dead. so I pulled it out, then without thinking, I put my disk in upside down, feeling what I thought was the click of the connectors mating. When I booted back up, all I got was a ? folder, so I opened it up, pulled out the disk, put it back in, same thing happened. then I had an awful thought that the SATA connector inside the MacBook might be broken, so I went over to ifixit and looked at how much new SATA connectors are, '$19, hmm, well if I'm going to have to, I'll have to' I thought. I decided then to follow the guide on replacing the cable. 21 screws later, the top was off and I could see the hard drive taunting me, 'I'm in the wrong way, you idiot!', so I flipped it back round, watched it connect into the SATA connector and put it back together. 5 minutes of reassembly and it was booting properly. And I wasted an hour because I didn't think to flip the disk over and try putting it in again.