I had this problem, but only with one device, my USB mouse. It happened no matter which port I plugged it into, and other USB devices - such as my keyboard or external hard drive - worked fine in all USB ports.
I tried a lot of different things, including most of the things suggested in this thread, but none of them worked. I was inspecting the USB connector on the cord to see if it was dirty, and compared it to the USB connector for my keyboard to see if one was dirty and the other wasn't. I saw that the metal part of the connector for the mouse was slightly bent outwards on one corner, and figured this may be causing it. I got a pair of pliers and squeezed that bent section back in. The connecter fit more snugly into the port and now the DNR problem was gone.
I think for most people this is not the problem, since most are saying that the problem is for all devices and in all ports. Some people have posted saying it's only for one device though, and in those cases you may want to compare the USB connector to a USB connector for another device, and see if the one that isn't working is bent or broken in any way. If it is, you may be able to use pliers to squeeze the dent in or pull it out.