I just spoke with Support on this... (Caller 50 in Queue when I started waiting). They claim there are 4 Million drone users and a large percentage of them are all trying to upgrade at the same time. Their network cannot handle the load for the drone firmware. Problem is, the RC will upgrade quick and easy and leave me half way there. The Drone is a whole different story. Their advice, keep trying until you get through and get the download complete. This means power cycle the drone and restart the upgrade again. It is easier with DJI Assistant 2 (strange version 1.00.002)
Wish we could download (slow via FTP or HTTP) and place it on a MicroSD Card.
We are all stuck in the same boat. DJI needs more servers and better infrastructure to deliver the firmware updates. Since the DJI go alerts you on start, and the RC was quick, we all get suckered into this problem.