Hi!
Have you managed to solve this issue? I'm having exactly the same issue, but with ex3300-48t switch and the same 12.3R6.6 junos.
I followed all the manuals on juniper site considering this issue, but no help. Rebooted 3 times already, still boots from backup.
The commands showd in previous messages not working with ex switches:
request system snapshot media internal slice 1
^
syntax error, expecting <data>.