When did you unplug the USB receiver? Before booting up? While in the editor?
Also using Window 7 (SP1 beta). Whenever I want to upload or download to the MX-900 I first unplug the Logitech Unifying receiver USB which is plugged into a USB hub port on my HP display. I then plug in a "retired" Microsoft USB mouse. Once the mouse pointer appears I start the MX-900 Editor using the Run as administrator option. Once the editor opens I connect the remote to a USB port. At the point the app to remote communications are perfect.
I also have to employ the mouse runaround whenever there are URC software/firmware updates. Years ago I tried to resolve this signals interoperability but to no avail so I'm not going to waste anymore time on the problem.