I had a rather interesting machine come in for service last week. The customer in question had purchased a new MacBook Pro 13-inch with Touch Bar in 2016 with the release of the new models. He hands me the computer with a bag of USB drives. The computer works fine, but all four drives are completely dead.
The computer had apparently been toasting them, and any USB device connected to the machine would get extremely hot and fail within seconds of being connected. Not something I have seen before with a Mac, and presumably not something the other repairers have seen before either. Apple themselves weren't able to shed any light on the issue, and another repairer, unable to replicate the fault at all, went as far as to suggest it was potentially malware causing the issue. Not even remotely close.
So I accepted the machine in for service. Now Apple doesn't have a diagnostic tool for such issues. If you plug a device in and it blows up, it blows up. You're no closer to knowing why. So I had to fashion a tool - a defective USB keyboard had its connector hacked off and a multimeter attached to the positive and negative wires, then connected through a USB-C to USB-A passthrough to the computer.
5.17v. Within the USB specification, so no trouble here. All four ports tested the same.
But the MacBook Pro uses USB-C charging. Being different from any previous Mac produced before it, with the exception of the 12" MacBook of course, this seemed like it could be our smoking gun. I connected the Apple USB-C power adapter and...
7.55v. Well outside of the maximum voltage of 5.25v as allowed by the USB specification. All four ports tested the same while the charger was connected to the rear left side port.
What happened was due to a fault somewhere on the Logic Board, power from an external source was being allowed to enter the USB power circuit while the computer was charging. I say an "external source" because the 87w USB-C power adapter we used for testing is supposed to deliver 20.2v / 4.2A, 9V / 3A or 5.2v / 2.4A. It's possible the bridge had occurred somewhere upstream where it's stepped down to 7.55v. Still well outside of specifications, no matter how one slices it.
Keep in mind that despite the computer delivering a significantly higher than normal voltage, the machine didn't turn off or exhibit any abnormal behaviour. Not even so much as a USB device warning. The Mac has sensors for detecting an overcurrent situation, but nothing to detect for or protect against overvoltage.
With a replacement of the Logic Board, the computer went home fixed. Finally.
But I've never come across an issue like this before, at least not with an Apple computer. As we enter the world of the all-purpose connector that is USB-C, I expect to see more of this kind of issue though, and a lot more dead devices. Hopefully as the standard develops and later revisions of Logic Boards are introduced in newer models, there will start to be more safeguards against this kind of fault.
I just had to share this one with everyone. I'm no stranger to difficult, obscure and unusual faults, but this was one of the more unusual ones I've come across in recent times, for sure.