During firmware flashing on your Original Prusa MK4/MK3.9, you might get the message "USB drive not connected! Please insert a USB drive with a valid firmware file".
Error name: USB not connected
Error code: #13602 (MK4) #21602 (MK3.9)
This issue happens when there is an error reading the USB drive.
Make sure that the USB drive is connected to the printer, and that the USB port in the mainboard is not damaged. If the error persists, try using a new, FAT32-formatted USB drive.