I'm trying to upgrade my Phantom 4 firmware.  First step is to open DJI Assistant 2 on a computer then connect the P4 to the computer using an Micro-USB cable.

I've tried this on both my Mac and Windows PC and and tried several different USB cables and it doesn't work so I am beginning to wonder if the USB connection on the P4 is defective.  Anyone how how to troublesheet this?

I too had issues with Assistant 2 (Ver. 1.2.0) connecting to the P4 Std. with Windows 10-64x.  Even the two iPads had issues trying to do one of the NFZ updates.

The only one that worked - and  it usually doesn't work - was my Android phone.

More odd, when I used the same Assistant 2 above to update the Inspire 2 it did work right for both the firmware and NFZ both.  You might be right that there is something wrong with the firmware in the P4 that keeps it from updating to it.  Sometimes it downloads to the computer for the P4, but it refuses to install into the drone for whatever reason.

I had similar issues with several versions of the assistant with 64bit Win 10. Through much fiddling around , I was able to finally get it  going by using device manager and physically deleting the devices and driver software and reinstalling the Assistant software. Like so much  of the software, it seems the DJI drivers were simply not adequately tested prior to release. You may have some luck using the above process. I also had some success using an entirely new installation of Assistant on a notepad which I now use for any updates  but I realise that this may not be an option for everyone.