One of the challenges of troubleshooting is that the same symptoms can have multiple causes. Another is that failing components can cover up or mimic symptoms of other failing components. This is often called masking. We had a MacBook Pro come into the shop that, when powered on, got to the grey screen; the apple never appeared on the screen, and the machine never fully booted. With a symptom like this, the first thing that comes to mind is the hard drive, and the initial diagnosis seemed to bear this out.
The machine passed Apple’s service diagnostics except for the hard drive tests. In fact, the hard drive tests weren’t even available, meaning that the diagnostic tool didn’t even recognize that there was a drive to test. Pulling the drive and testing it outside the machine confirmed that the drive had an abundance of bad sectors, a sure sign that it needed replacing.
Running the Apple diagnostics also showed that the machine would boot from a known good volume—the test suite boots the machine with a version of OS X via one of the USB ports. With this information in hand, the technician ordered a replacement drive. But when the drive arrived the next day and was installed, the machine still refused to boot. The original symptom persisted: it booted to a blank grey screen. It was obvious at this point the problem went beyond the hard drive.
After replacing the interconnect cable between the drive and the logic board and getting no change in the machine, a logic board was ordered. After installation the machine booted normally and was returned to the customer. This repair perfectly illustrates the concept of masking. In this case, even though the symptoms were classic for a failing hard drive, the real reason the machine would not boot was a logic board failure. The situation was further confused by the fact that the drive need replacing as well.
As an aside, at pickup time the customer asked a question we get a lot in these kind of cases: Why does my new machine have two bad parts? Understandably, this can cause people some concern. After all, one of the reasons people choose Apple is the high reliability of the hardware. The answer is that the machine is an interconnected system, not a collection of separate parts. When a part fails, it can cause serious damage to other parts in the machine. In the logic board/hard drive interconnect there is also a power circuit in addition to the data circuits, so there is even more chance for a failure such as a short circuit to cause problems. In this case we would never be able to say which went first, but in my experience the failure of one of these components almost certainly caused the failure of the other, necessitating the replacement of both. We call failures of one component due to another “cascading failure.”