One morning my Mac decided not to boot. I tried all the restart options to no avail. I was not able to run diags. I took the drive out and tested it on another Mac. Disk Utility could not repair the disk. I used a data recovery tool to get most of the data off the disk, reformatted, and reloaded OSX. I booted the Mac from the USB drive and all was well, or so I thought. I put the disk back into the Mac, but the Mac would not boot. I ran the Mac Diags and it reported no errors? After more Google searches I saw that others had had similar problems and a drive cable was the problem. I searched for the part on iFixit and confirmed that others had similar problems so, I ordered the drive cable from iFixit. I figured that $60 was less than the diag cost at the Apple Store and way less than a new logic board. After installing the drive cable, the Mac booted up fine and my problems are resolved!
The repair was very easy. Replacing the drive cable on the Macbook Pro Unibody Mid-2009 did not require any special tools. The cable even comes with new adhesive. I am very pleased with the repair and the quality of the iFixit parts and instructions.
BACKUP YOUR DRIVE EVERY DAY! Unfortunately, I was a bit too lax with my backups, so I had to spend $80 on recovery software and had to spend a lot more time recovering my data.