On modern Windows systems, there is a reported incompatibility with the virtual comm port that causes the computer to not see the radio. If you’ve already attempted using a different USB cable, different USB ports, and you’re using the top USB port on the radio- it’s possible that the driver incompatibility is the issue. Using Windows 8 VCP drivers on the virtual comm port should resolve the issue.
The links to which could be found below.
STSW-STM32102 - STM32 Virtual COM Port Driver - STMicroelectronics