The article describes when backup partition gets copied to primary partition on SRX devices.
> show system snapshot media internal
node0:
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Information for snapshot on internal (/dev/da0s1a) (primary)
Creation date: May 22 15:28:10 2025
JUNOS version on snapshot:
junos : 23.4R2-S4.9
Information for snapshot on internal (/dev/da0s2a) (backup)
Creation date: May 14 15:25:35 2025
junos : 21.2R3-S7.7
> request system snapshot slice alternate node all
Formatting alternate root (/dev/da0s1a)...
Copying '/dev/da0s2a' to '/dev/da0s1a' .. (this may take a few minutes)
The following filesystems were archived: /
This happens if the snapshot command is run before the reboot of device after adding the Junos software.