Have you tried a different usb port on your computer?

On my laptop for example, only one port will work with the MX-3000, whereas both ports will work fine with my PDA and phone etc.

I also had a previous laptop (Gateway), that we had to replace since it point blank refused to communicate with some MX-3000's even after trying different versions of activeSync and spending a couple of hours on the phone with URC tech support!