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Strange issue. I have a 2012 iMac running OS X 10.11.6 (which I don't think is relevant). Hard drive failed maybe a year ago and was replaced. Runs great except, the 2 USB ports seem to alternate not working. For a while they both work, then only one or the other, but not the SAME one or the other. Ideas? Thank you!
iMac 21.5″, OS X 10.11
Pay attention to the devices you are connecting to those ports. I've had a USB3 hard drive disable a USB port rather consistently. A reboot was required to re-enable the USB port. I finally ended up using a good brand of powered USB3 hub with UASP support which stopped the USB3 hard drive from disabling the USB port on the Mac. It may be something you want to consider as well.
I would also like to recommend getting and using a self-power USB hub with your 2012 iMac.
If you have been hot-plugging USB devices a lot, then it is possible that you have a worn, loose or faulty port that is shorting or grounding out. With a self-power hub, you can hot-plug into the hub and eliminate any further ware-n-tare to the iMac's USB ports.
It is also possible that you have a bad USB cord or device that is causing the port over-load protection to trip off.
Thanks for the input to you both. I have a powered USB hub and have been using one for years. Often it's the port into which the hub is plugged in that shuts off. I switch to the other port and it works again. Until that one stops, then I switch to the former one and I'm back in biz. It's weird.
Still sounds like a bad USB cable or faulty USB device.
I would,
alternating USB malfunction
