This is for my notes. Generally I use this to upgrade the firmware of an EX4200 switch to the same JunOS version as the rest of a virtual chassis, BEFORE connecting the VC cables. I'm doing this from memory, so if there are any mistakes let me know.
You will need to have access to a shell. The "root" user logs into shell automatically, other users should run "start shell"
From the shell run these commands:
# mkdir /mnt/usb
Insert the USB stick (FAT32 formatted) into the port at the back of the switch. It should have the firmware on it, generally in a file named something like "jinstall-ex-10.3R3.4-domestic-signed.tgz"
# mount -t msdosfs /dev/da1s1 /mnt/usb"
Note that tab completion works on Junos, to save some typing just enter the frist few characters of the filename and hit "tab"
# cp /mnt/usb/jinstall-ex-10.3R3.4-domestic-signed.tgz /var/tmp
If the switch is running firmware older than 10.3 you should upgrade the loader too:
# cp /mnt/usb/jloader-ex-3242-11.3I20110326_0802_hmerge-signed.tgz /var/tmp
# umount /mnt/usb
At the ">" cli prompt:
If the firmware on the switch is older than 10.3 you will need to install the updated jloader
> request system software add /var/tmp/jloader-ex-3242-11.3I20110326_0802_hmerge-signed.tgz
> request system software add /var/tmp/jinstall-ex-10.3R3.4-domestic-signed.tgz reboot