While flashing firmware to your Original Prusa MK4/MK3.9, an error might occur, and the screen will display the following message: "Error erasing flash! Restart the printer and try again".
Error name: Flash erase error
Error code: #13605 (MK4) #21605 (MK3.9)
This error is shown when the printer has failed to delete the internal flash memory, so the new firmware can't be flashed.
Download the firmware and if necessary the bootloader, and transfer it to your USB drive. Make sure to unmount the drive from your operating system before physically removing it.
Insert the USB drive back into the printer and reboot the printer using the reset button directly next to the knob to initiate the flashing procedure. In case the procedure does not start, reboot the printer again by pressing the same reset button, and while the printer is booting up, press once the rotating knob.
If the issue persists, the problem might be in the USB flash drive. Find any other USB drive in your household, make sure it is formatted to FAT32, copy both firmware files, and repeat the previously described procedure.
Access the xBuddy board by loosening the four screws in the below picture, and inspect it for any visible damage.