A few days ago, I noticed that my Mac, while running Mountain Lion, would suddenly freeze and start beachballing, even when not in use. There are no aftermarket components or peripherals. I had to restart it 5-6 times in one day and half the time it would boot up, the other half it would hang on the gray screen and I'd have to hold the power button to shut it down. I tried booting up in safe mode, which worked the few first times, but then even booting up in safe mode became imposible. I reset the VRAM/PRAM and that did absolutely nothing. Now the device consistently refuses to start and ALWAYS displays the gray screen (doesn't even load the apple logo). I removed the back plate and noticed that the HDD makes a clicking noise as soon as it receives any power. I believe there may be a mechanical failure. Here are some more bizarre details: I have a bootable external hard drive. When I disconnect the internal HDD and connect the external hard drive, I am able to hold "Option" while powering on the Macbook Pro and the external hard drive will show up as a bootable drive. However, if I leave the internal HDD connected, holding "Option" does nothing and the computer will not "see" the external hard drive. This is bizarre because I had previously installed MacOS on my internal hard drive using my bootable external hard drive, so it doesn't make any sense for the ability to detect the external hard drive to be affected by the presence of the internal HDD.