I had the exact same problem today. Tried to update firmware on my Avh-4000nex from 1.14 -> 1.20.
the update stopped at exactly 66%. This happened from both USB drive and SD card.
recovered by rebooting with 1.14 files on thumb drive when rebooting.
anyone successfully update this unit to 1.20 firmware?
I also tried updating from 1.14 -> 1.15 and had exact same result. The update crashed at 66% complete.
bummer.