Failed firmware upgrade - can't revert
I have a MK3S which I recently attempted to upgrade with the MMU2S kit. After completing assembly, I attempted to upgrade the firmware (Prusaslicer 2.2.0 on Ubuntu, also tried Windows 7). The flash went just fine to approximately 50% (100% written and 100% verified according to the display on the LCD) and then fails, with the "External SPI flash W25X20CL not responding" error. This error now presents every time I start up the printer. I have attempted to re-flash with various other windows and Linux machines, always failing at the same point. I have found this article and attempted to flash with the LCD cables disconnected with no difference in outcome. I have also attempted the factory reset procedure to bring back my MK3S firmware and try from a blank slate, however that instantly produces the aforementioned error.
I have gone over every troubleshooting step I can find, the printer is an MK3S that was ordered in May and was working quite well before this firmware attempt. Are there any other troubleshooting steps to try?
Sounds like your printer would still be under warranty, best contact Prusa and see if you can get a replacement board
This means your flash chip (where languages are stored) may not be working. If not under warranty and/or you can live with English only, you can ignore it and continue using the printer.
If under warranty, contact PR and they will probably replace the board.