USB Device Disabled with none plugged in

Hey guys

Apple support have been unable to help me so far with them.

Updated a mid-2010 iMac, I downloaded & installed 10.11 and it went well (the first time), this was a clean install.

I then go unplug everything, leaving just the power cord plugged in.

Still get the "USB Device Disabled".

I close it, 2 seconds later its back.

I've reset the PRAM (or whatever it is now), unplugged the power cord for 30 seconds and plugged back in, still.

I went back down to 10.10, it doesn't happen.

I have lost 2 USB ports that seem to have died, that was at the start of the year and haven't had an issue with the other ports since.

Apple support can't work it out and the last thing I want is this message there all the time.

(With each release of their OS I'm getting closer to going to PC)

Thanks