I'm trying to install RetroPie on an USB stick and I want to boot on it. So I follow this instructions HOW TO CREATE A RETROPIE USB BOOT DRIVE FOR RASPBERRY PI 3
The start of the instructions seems standard (I've seen this on other sites, for booting on USB, not specifically for retroPie), but It doesn't work for me.
sudo apt-get update && sudo apt-get install rpi-update sudo BRANCH=next rpi-update echo program_usb_boot_mode=1 | sudo tee -a /boot/config.txt
after this, the file /boot/config.txt ends with the sentence, but when I type sudo reboot (I tried two times), after reboot the Pi is blocked, just after the launch screen. The red led is on, and the green one blinks. I have a Raspberry 3B+, I don't understand what's the problem.