Further to my post in August copied below I have determined what I believe is a bug in the USB 3 driver. By removing the driver and operating in USB 2.0 mode everything works just fine.
I can understand why Intel may not have discovered this anomaly since it is only present after USB devices have been operating for a long period and are then completely powered down or totally disconnected (unplugged)
NUC D54250WYKH: USB 3.0 driver bug, Hardware design bug or Simply broken?
I am using the aforementioned board as a dedicated music server and almost everything is working as expected. The problem I have is that the board consumes 30% CPU power when there is nothing connected to one of the four USB 3.0 ports.
In operation one USB port is connected to an external audio DAC and the other 3 ports are unused. While the DAC is active everything works fine with the CPU idling between 2% and 12%. The phenomenon shows up when the DAC is either powered down or unplugged after several hours use. The CPU jumps to 30% and remains there until either the DAC is powered back or I connect something to activate one of the ports
A workaround for the moment is to leave a thumb drive plugged into one of the other USB 3.0 ports but clearly this is an unacceptable solution.
The operating system is Win 7 Professional and all drivers and latest bios (27) are installed.
I would like to emphasise that this anomaly only shows itself after the system has been playing music for several hours, rebooting the system without anything connected to the USB ports also seems to bring the idle power back to normal.