EX3400 is boot looping. The kernel cannot be found, reinstall is required.

Power off the EX3400

1) Download the appropriate image for the device (ex: "junos-install-media-usb-arm-32-15.1X53-D59.4-limited.img.gz")

2) Extract the image

3) Write the extracted image to a USB-device using dd with bs=1m or bs=1M depending on version

4) Insert the USB-device and power on the EX3400

5) Hit 5 for [M]ore options and 5 again for [B]oot prompt

6) Run lsdev and confirm device disk1s1a exists

7) Run set currdev="disk1s1a"

8) Run include /boot/loader.rc to reboot the device

9) Wait for the installation to complete - be patient.