Hello ,
I can give you an easy and fast way to recover . If you have another SRX3400 in your network , connect a USB to that device , take a snapshot using command :
> request system snapshot media usb partition
Now once its complete , connect the USB to the faulty SRX3400 and reboot or power cycle . It will try to boot from USB . Once its booted up , copy the snapshot to SRX using
> request system snapshot media internal
Let us know if this is possible .