This article describes how to perform Format Install by using the "install " command on legacy Juniper Networks SRX 300 Series

1. Stuck in >loader prompt
2. Stuck in boot loop
3. Can see the error: can't load kernel

Method Using a USB Device :

1. To perform this recovery installation, the USB device should be formatted to FAT-32 and should be empty (recommended USB size: 1GB, 2GB, or 4GB). Review the complete USB compatibility specifications listed in Size of USB drives recommended for SRX device upgrade/downgrade (juniper.net)
2. Copy the Junos OS package to the USB device.
3. Power off the device
4. Plug the USB device into the device
5. Accessing the Loader Prompt, to access the Loader prompt use a console connection to connect to the device.  
6. Reboot the device. During the reboot, the following message is displayed:  
   1. Loading /boot/defaults/loader.conf  
7. After this message appears, see the following prompt:  
   1. Hit [Enter] to boot immediately, or the space bar for the command prompt.  
   2. Press the space bar to get to the Loader prompt.  
   3. If you are seeing this message: Press Enter to stop auto boot sequencing and to enter loader prompt, Then Press Enter 
   4. The loader> prompt appears.  
   5. Loader>install file:///<package_name.tgz-on USB>  

An example of the above command is as follows :

loader> install file:///junos-srxsme-10.0R2-domestic.tgz  

This formats the internal media and installs the new Junos OS image on the media with dual-root partitioning.