Sounds like your USB drivers are corrupted. Try a fresh install, but uninstalling all and any copies of DM and your devices OS on the PC side. And do both installations over again. This has happened to me before. Use this for the uninstall process as it will delete the .inf files in the registry. Its a great little free program.

BUT, I would first rule out the cable, by trying a different cable and rule out the actual USB port(s) on the PC by trying a different port AND trying another USB device.

Hope this helps.