WUDFRd is a service and it is called "Windows Driver Foundation - User-mode Driver Framework".
I went into services.msc and looked at Windows Driver Foundation - User-mode Driver Framework, startup type was set to "Manual".
I changed the startup type to "Automatic", shut down the PC, restarted, checked to see that it was still set to automatic and that it was started. Plugged in a USB and no error code, plugged in another USB and no error code. Shut down, started again, checked again and no error code.
I have three PC's with Windows 7 and I looked at another one this morning. It had just started throwing this error code on Monday, the service on this PC was also set to manual, changed it to automatic and no more error code. I looked at my third PC this evening and the service was set to automatic.
I have no idea why, but somehow on two out of three Windows 7 PC's this service got changed from automatic to manual last Friday. I used one on Saturday and that is when the error code started on that one. I used the second one starting on Monday and that is when the error code started on that one.