I have had the subject issue with my external HDD for some months now. I figured it was a usb port problem on my motherboard so yesterday I installed a new one but still have the same problem. I have talked to both Microsoft and WD but neither have been able provide a solution. As it stands I have to disconnect both the power and usb cables from the HDD and reconnect them to get the PC to recognize the HDD. I connected the HDD to another pc and to check if the problem would show up on that pc but the HDD is recognized properly on that pc.