I can't boot on my usb device

I copy to my USB device the OS from the emmc memory of the board, then i modify the .conf on the emmc and on the USB with the PARTUUID of the USB device but when i reboot, the board boot always on the emmc. Please i need help ! How can i boot on the USB device ?

Please check

https://docs.nvidia.com/jetson/archives/l4t-archived/l4t-3271/index.html#page/Tegra%20Linux%20Driver%20Package%20Development%20Guide/flashing.html#wwpID0E0HO0HA