You will find the require steps to proceed with USB upgrade. 

Before you install or upgrade Junos OS, you should ensure that some basic checks, such as having sufficient disk space and backing up configurations, are in place.

Checklist for reinstalling Junos OS

• Registering the active configuration (Junos OS): show configuration | save {filename_for_backup}
• Backup the active and running file: system request system snapshot (not accepted by all SRX)

Validate there are enough space on the device to proceed with the upgrade:

• show system storage

If you require more space to proceed with the upgrade, please refer to the documentation: 

request system storage cleanup (Junos OS)

request system storage cleanup (Junos OS Evolved)

• Proceed to mount USB:

Enter the shell as the root user:

user@switch> start shell user root

Password:

root@switch% 

• Before inserting the USB device, perform the following:

root@% ls /dev/da*

/dev/da0 /dev/da0s1c /dev/da0s2a /dev/da0s3 /dev/da0s3e

/dev/da0s1 /dev/da0s1f /dev/da0s2c /dev/da0s3c

/dev/da0s1a /dev/da0s2 /dev/da0s2f /dev/da0s3d

• Insert the USB drive in the USB port. The following output will be displayed:

root@% umass1: TOSHIBA TransMemory, rev 2.00/1.00, addr 3

da2 at umass-sim1 bus 1 target 0 lun 0

da2: <TOSHIBA TransMemory 5.00> Removable Direct Access SCSI-0 device 

da2: 40.000MB/s transfers

da2: 983MB (2013184 512 byte sectors: 64H 32S/T 983C)

Note : " /dev/da2s1 " is the USB drive. If the console session is not available while inserting the USB, check the /var/log file named "messages" for logs related to "da" (for example, show log messages | match da ). The same four lines will be logged as is shown on the console if it is plugged live.

Create a directory for the USB drive to mount to:

root@% mkdir /var/tmp/usb

Mount the USB drive to the /var/tmp/usb directory:

root@% mount_msdosfs /dev/da2s1 /var/tmp/usb

root@% ls /var/tmp/usb

images.tgz

juniper.conf.1.gz

Note : " ls /var/tmp/usb " will show all the files in the USB drive.

If you encounter the mount_msdosfs: /dev/da0: Invalid argument or the mount_msdosfs: /dev/da0s1: Operation not permitted errors during this step, refer to Possible Issues While Mounting USBs.

• Copy the image from the USB to the device: 

root@% cp /var/tmp/usb/images.tgz /var/tmp

root@% ls /var/tmp

images.tgz

In this case, the images.tgz file from the USB is successfully copied to the /var/tmp directory.

• Copy the backup files from the device to the USB:

 root@% cp /var/tmp/{filename_for_backup} /var/tmp/usb

root@% ls /var/tmp/usb

{filename_for_backup}

In this case, the {filename_for_backup} file from the device is successfully copied to the USB.

• Unmount the USB device after the file has been copied:

root@% umount /var/tmp/usb

root@% umass1: at uhub0 port 1 (addr 3) disconnected

(da1:umass-sim1:1:0:0): lost device

(da1:umass-sim1:1:0:0): removing device entry

umass1: detached

Note: ​Ensure that USB devices are unmounted properly before removing them from the router. For devices with multiple USBs, make sure that you unmount both devices.

As documented in PR583332 , ​removing USB drive unsafely (removing without unmounting) can cause kernel crash.

• Start with the upgrade process:

request system software add /var/tmp/images.tgz no-copy no-validate 

• Once the upgrade has been completed, proceed to reboot the device.

request system reboot

• If there are no errors booting the device, please make a quick health check of the device using the following commands:

show system core-dumps

         >>> Verify if any recent cores.

show chassis alarms

         >>> Verify if any Active alarms.

show chassis routing-engine

         >>> Check the State/CPU/Memory/Last Reboot Reason.

show chassis hardware

show chassis fpc

show chassis fpc pic-status

         >>> Verify if all Hardware components are listed and online.

        >>> Verify if all the storage device(s) are listed.

show system processes extensive | no more

        >>> Do a quick glance on running processes, more checks on this will follow later.

show chassis environment

show chassis power

show chassis environment pem

        >>> Verify Total Power Available/Allocated Power/ Whether All PSMs are up and Feeds are connected.

show system storage

        >>> Check the storage mounts and see if /var is mounted properly. Also check for free space.