While flashing firmware to your Original Prusa MK4/MK3.9, an error might occur, and the screen will display the following message: "File system error! Try a different USB drive or format the drive with FAT32 filesystem (all data will be lost)".
Error name: File system error
Error code: #13613 (MK4) #21613 (MK3.9)
The printer shows this error when it can't read the USB drive.
Connect the USB drive to your computer and format it in FAT32. After formatting the USB drive, try flashing the firmware again.