Just a small note based on my personal experience. At first, shorting the two "Erase" pads as shown in the photo did not work...
Then I realized that maybe the pads could have been covered by a transparent layer of protective (and insulating...) paint.
I disconnected the unit from the USB port, and gently scraped the top of the two pads so to expose the bare solder.
Then I reconnected the unit to the USB port, and repeat the shorting procedure.
This time everything worked as it should, and was able to correctly flash the firmware.
73 Alberto I2PHD
Credo Ut Intelligam