I am new to this forum, but I have been to this site more often to get advice about various Mac issues. One of my friends came to me with his MacBook Pro (15 inch, early 2009) which did not want to boot up anymore. Every time he started the computer, it would hang at the grey Apple logo. So he brought it to the Mac shop and they told him his HDD and PowerDrive was bunk and so he bought a new one. Now the issue was that the new HDD also did not want to boot up in his Mac after installing OS X on it from another computer. So I took out his old HDD and checked it on my Mac under Disk Utilities and saw that it needed to be repaired. After repairing it, it still did not want to boot up from the old HDD. Strangely, when we put his old HDD in an external enclosure, it did work and the computer booted up fine, albeit quite slow.
I believe that the HDD ribbon is broken, but I cannot say that this is true 100%. After checking AHT no errors popped up (although I tried to do an extended check the first time and it froze... The second time no errors popped up). I also tried replacing the RAM, but still the computer would not book up properly. Is there a way in checking the HDD ribbon via a multimeter or something?