I also have this box, but can't upgrade firmware.
Could someone tell me more precisely what is the procedure?
For the moment, I keep the reset button pressed, unplug power supply, plug it back, and when I see the light blinking three times, I release the reset button, and plug the SD card.
Result is that I see the message "can't mount /dev/sda1", and the factory reset is performed.
I have the image files on my sdcard.
What might be wrong in what I'm doing?