Instructions are written poorly. They state,

Use one of the following:

- USB Flash Drive (for AVIC-5100NEX, AVIC-6100NEX, AVIC-8100NEX and AVH-4100NEX) formatted to FAT32 and 2GB or larger in capacity OR
- MicroSD card (for AVIC-7100NEX, AVIC-8100NEX and AVH-4100NEX) formatted to FAT32 and 2GB or larger in capacity

Either storage device should be blank without any files on them.

Which USB port to use: 
IMPORTANT: Only USB Port 1 on your NEX head unit should be used to update the firmware.

.. Now is the USB 1 port only in referance to the USB drive method or is it being assumed that you will connect the SD card into a USB reader?

Can the firmware be installed using the SD card in the SD slot behind the screen? 

Thanks.

They should have added

Which USB port to use if you are using a USB Flash Drive:

I updated my NEX using just the SD card slot behind the screen.  My USB Port 1 is used to power my Chromecast.

Thank you