When flashing the firmware on your Original Prusa MK4/MK3.9, you might get the message "Firmware file missing in the USB flash!".
Error name: No FW on USB flash drive
Error code: #13604 (MK4) #21604 (MK3.9)
This error happens when you attempt flashing firmware on the printer, but the printer does not find any firmware in the USB drive that can be uploaded to the printer.
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.