What happened?
While flashing firmware to your Original Prusa Printers, an error might occur, and the screen will display the following message: "Firmware file has invalid size! Check the file on the USB drive and try again".
Error name: Invalid FW size on USB
Error code: #26603 (MK4S) #13603 (MK4) #27603 (MK3.9S) #21603 (MK3.9) #28603 (MK3.5S) #23603 (MK3.5) #31603 (CORE One)
This error can happen if the bootloader file is edited, or if the USB is removed from the computer before the end of the file transfer.
How to fix it?
Delete the firmware file from the USB drive, and download it again. Transfer the file to the USB drive. Make sure to unmount the drive from your operating system before physically removing it. This will prevent the USB drive from being removed too early.
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.