Hello CP1,
I just have an SRX345, but I think the behavior is the same as SRX550. When you have look at the following website
https://www.juniper.net/documentation/en_US/junos/topics/reference/command-summary/request-system-snapshot-partition.html
"media internal" is the default option. When you execute this without the "slice alternate" option, you can see this:
> request system snapshot media internal
node0:
--------------------------------------------------------------------------
error: Cannot snapshot to current boot device
So you need the "slice alternate" option to make a snapshot to the backup partition, which is not in use:
> request system snapshot slice alternate
node0:
--------------------------------------------------------------------------
Formatting alternate root (/dev/da0s1a)...
Copying '/dev/da0s2a' to '/dev/da0s1a' .. (this may take a few minutes)
The following filesystems were archived: /