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How to Fix Mac USB Ports When it Stop Working

The Solution to No-Cause Random USB Failure on a Mac: Resetting SMC

So, you’re confident a USB port is the problem and not the device itself? The first thing you should do is reset the Mac SMC, a simple process which is slightly different depending on the Mac hardware.

You can learn how to reset the SMC on all Macs here in our detailed tutorial. For those with a MacBook, MacBook Pro, and MacBook Air, all you need to do is this:
  • Shut down the Mac and connect the power cable
  • Hold down Shift+Control+Option+Power concurrently for a few seconds, when the light on the power adapter blinks or changes colors you’ll know SMC reset is complete
  • Release all keys at the same time
  • Boot the MacBook Pro / Air by hitting the power button as is typical
The USB ports on the MacBook should now work just fine, which you can confirm by connecting the USB device as usual and checking to be sure that it works – if it’s still not working, try another USB port, and try another USB device.

If, after an SMC reset, none of the USB ports are working, and no USB device is working, something else is likely up the Mac

Other Considerations for USB Not Working on a Mac
  • If a single USB port has failed but other USB ports are continuing to work on the Mac, even after an SMC reset, there may be an issue with that specific hardware USB port – you can either avoid using that USB port and instead use a USB hub for one of the other functioning ports, or you can have Apple run a hardware diagnostics test on the machine to determine if there is any hardware failure
  • If liquid has splashed on the Mac or the MacBook has contacted water, you will need to take immediate steps to power down and dry out the Mac for a chance to prevent permanent damage – liquid contact can destroy a Mac rather quickly, or damage certain hardware functionality, like USB
  • If some USB devices work on the Mac, but another USB device does not work, consider that the device may be incompatible with that particular Mac, the USB device may be failing or have failed, or, in some situations, the USB device may need specific hardware drivers from the manufacturer in order to work properly (typically the case with special use devices, high-end scanners, etc)
  • If there are firmware and OS X software updates available, install them onto the Mac. You can check this by going to the  Apple menu > App Store > Updates section – be sure to backup the Mac before installing any system software
Source: http://osxdaily.com/2015/08/09/mac-usb-ports-not-working-fix/