Hello Frank,
As EX3400 is not able to load kernel it does not go to OAM volume which can be used to install junos from bootable USB.
Create bootable USB with install media image 15.1X53-D57 version(junos-install-media-usb-arm-32-15.1X53-D57.3-limited.img.gz) and insert usb to switch.
Please follow recovery instructions shown below. Install --format command should not be used for EX2300/3400. We are creating a KB for this procedure.
Type '?' for a list of commands, 'help' for more detailed help.
loader>
- Select USB manually
loader> set currdev="disk1s1a"
2. Manually boot from USB:
loader> include /boot/loader.rc
3. you will observe following messages and switch will boot from USB:
/boot/kernel data=0xe48000 syms=[0x4+0x5cdb0+0x4+0x81971]
/boot/dtb/ex3400.dtb size=0x1f25
/boot/contents.iso size=0x171e800
Hope this will be helpful for recovery.
Regards,
Raj Maradia