There’s a few ways to do this. Here’s the easiest:
First, format your USB drive as fat32. Download the compressed image from Juniper (e.g. junos-srxsme-15.1X49-D80.4-domestic.tgz) and load it onto the USB drive.
Whilst logged into the SRX’s console, plug in the USB drive. You’ll see something like the below:
da1 at umass-sim1 bus 1 target 0 lun 0
da1: <TOSHIBA TransMemory 1.00> Removable Direct Access SCSI-4 device
da1: 40.000MB/s transfers
da1: 7400MB (15155200 512 byte sectors: 255H 63S/T 943C)
This tells you that the USB drive is available at /dev/da1. Assuming your drive has a single partition, that means the filesystem will be at /dev/da1.
Jump into the FreeBSD shell:
Create a directory and mount the USB drive to it:
mount_msdosfs /dev/da1 /var/tmp/usb/
Check that your JunOS image is present on the mount:
Jump back to the main JunOS console:
Install the image:
Reboot your device:
Once rebooted, sanity check your new install. If all is ok, you need to copy the JunOS image to the backup partition else if your SRX ever fails to boot then it will boot into your old JunOS:
Once done, you can check it:
Information for junos:
JUNOS Software Release [15.1X49-D80.4]
root> show system software backup
Backup JUNOS package information:
File name: /altroot/cf/packages/junos-15.1X49-D80.4-domestic
File size: 249978054
And that’s it.