Sounds like the device drivers are not loaded correctly. Have you looked at Windows -> Device manager -> USB Devices? Anything that says "myRIO"? i am not a myRIO expert but most USB issues are related to device drivers.
Additional;
Try a different USB port
Don't use a USB HUB
Do a full shutdown and restart PC, then when fully booted, plug in USB to mtyRIO
Hope this helps,
Matt
Matt Fitzsimons
NI Alliance Member
LabVIEW Champion
NI Certified LabVIEW Architect
LabVIEW, LV-RT, Vision, DAQ, Motion, and FPGA