I can’t answer in the Official forum but thought I would throw out a few things you can try that may (or may not) help.
You seem to be trying to update master firmware using ROBOTC. I almost always try and use the VEX update utility to do this, you may try installing and using that as an alternative, it’s available here.
Software downloads
The cortex can enumerate with the PC as two different types of device. When being used to load user programs, it is seen as a communications class device (ie. a COM port) the vex driver provides this functionality. When running in boatload mode it re-enumerates as a human interface divide (HID), some PCs have an issue with this re-enumeration, for example, I have very little success doing firmware updates running windows with a virtual machine. I have found that using a USB hub can help with this, you may want to try that, I know it doesn’t make any sense but that’s what I’ve found through trial and error.