While flashing firmware to your Original Prusa MK4/MK3.9, an error might occur, and the screen will display the following message: "USB flash drive contains unsupported firmware BBF file".
Error name: Unsupported BBF version
Error code: #13614 (MK4) #21614 (MK3.9)
This error happens when the BBF header is not compatible or has been changed.
Download the firmware and if necessary the bootloader, and transfer them to your USB drive. Do not make any edits to the files other than unzipping them. 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.