The command is basically archiving whole stuff (configuration file and juons image) from the partition on which it got boot image to alternate partition. Improper shutting down of device or hard restart may cause the image in primary partition to get corrupted and on next boot device will get boot image from secondary partition and at login following message will be displayed (*junos booted from secondary storage.Primary may be failed). Now solution is to archive the current boot partition to alternate partition which was earlier primary partition but image got corrupted due to some of the reason.
>request system snapshot media internal slice alternate
but even after that * same error message will be displayed again and again on every restart as juons has set the boot partition for itself from which it got boot image after primary privation got corrupted. Now issue following command
. >request system reboot media internal slice alternate
and the message will disappear
This concept is knows as Resilient Dual-Root Partitions and introduced in Juons 10.4R3. Actually idea behind is to provide resiliency on devices which got its boot image corrupted due to power outage and require serial console for juons re installation. Also visit http://www.juniper.net/techpubs/en_US/junos/topics/concept/switch-resilient-dual-root-partitions.html
Pleas mark this as accepted solution if it solves your problem
A Junos is good way of appreciation
Kashif Nawaz
JNCIP-Sec, JNCIS-Sec, JNCIS-Ent, JNCIA-Junos