Format the usb on any aother switch and copy the install file as indicated. Plug in the uSB to the EX Switch and reboot it. The bootup process will boot from the USB first.
Are you saying that when you press the reset button for about 15 seconds it does not work? if the answer is yes and the USB does not work for you, maybe a ticket with JTAC is order. Actually their is another method which I intend to try in a few weeks on a couple of devices. Take out the CF card from the affected switch, and put it in a USB cf reader. plug it in another switch and use the "request system snapshot media" to install the image on the card, then replace it.
If none works then you can use the procedure to install from tftp. that is well documented.
Try this one more time:
****Must have a console connection****
1.Reboot the system- Do not hit the spacebar at the first prompt!!
2.Look for “Loading /boot/defaults/loader.conf” then •Press the Spacebar immediately (if you wait for the prompt- it may be too late)
•loader> (if you set the environment you reinstall using tftp from this command)
•loader> boot –s (to access single user mode)
3.Enter recovery when prompted to go into recovery mode
/bin/sh: recovery
4.Enter “edit” and >Set root password
set system root-authentication plain-text-password
5.Commit the change and exit configuration mode—reboot