Posting here in case this helps somone.  Definitely an edge case...
TL;DR: Firmware updates fail if your local network subnet includes 192.168.42.x.


Attempting to update the firmware on my Mavic Pro from both DJI GO 4 and DJI Assistant failed.  In DJI GO the progress stalled at "Downloading 100%".  In DJI Assistant on my Mac never got past "Transferring 0%" and would fail with a generic error thereafter.

Then I noticed that the Mavic sets up a private network connection to communicate via USB, and has a DHCP server assigning 192.168.42.2 to the Mavic and 192.168.42.3 to the computer.  By coincidence my home LAN also uses the 192.168.42.0/24 subnet, so when the computer tried to connect to the Mavic's IP it was actually trying to connect to another host on my home LAN via the higher priority Ethernet interface.

To solve the problem you could try:
A. Go to a different network with a different IP address range.
B. Change settings on your router to use a different IP address range.
C. In DJI Assistant:
  1. Start the firmware update and wait until progress reaches 100% and switches to "Upgrading 0%"
  2. Disconnect your computer from your LAN by turning off WiFi or unplugging your Ethernet cable
  3. The update should continue and complete properly.  After you disconnect the Mavic you can rejoin your LAN.

Posting here in case this helps somone.  Definitely an edge case...
TL;DR: Firmware updates fail if your local network subnet includes 192.168.42.x.


Attempting to update the firmware on my Mavic Pro from both DJI GO 4 and DJI Assistant failed.  In DJI GO the progress stalled at "Downloading 100%".  In DJI Assistant on my Mac never got past "Transferring 0%" and would fail with a generic error thereafter.

Then I noticed that the Mavic sets up a private network connection to communicate via USB, and has a DHCP server assigning 192.168.42.2 to the Mavic and 192.168.42.3 to the computer.  By coincidence my home LAN also uses the 192.168.42.0/24 subnet, so when the computer tried to connect to the Mavic's IP it was actually trying to connect to another host on my home LAN via the higher priority Ethernet interface.

To solve the problem you could try:
A. Go to a different network with a different IP address range.
B. Change settings on your router to use a different IP address range.
C. In DJI Assistant:
  1. Start the firmware update and wait until progress reaches 100% and switches to "Upgrading 0%"
  2. Disconnect your computer from your LAN by turning off WiFi or unplugging your Ethernet cable
  3. The update should continue and complete properly.  After you disconnect the Mavic you can rejoin your LAN.