Since Junos 10.0, Junos supports dual-partitioned filesystems so that if the primary filesystem becomes corrupt, there’s a secondary with a copy of Junos in it to get the system to boot. There’s a good explanation of this here. Devices on Junos 9.6 and earlier have a single partition, but you can choose to change to dual partitions during an upgrade.
To find out whether your system is dual partitioned, issue the command “show system snapshot media internal” and you should see the following if dual partitions are in use:
root> show system snapshot media internal Information for snapshot on internal (/dev/da0s1a) (primary) Creation date: Mar 24 12:12:59 2011 JUNOS version on snapshot: junos : 10.4R1.9-domestic Information for snapshot on internal (/dev/da0s2a) (backup) Creation date: Mar 24 12:14:29 2011 JUNOS version on snapshot: junos : 10.4R1.9-domestic root>