Posting this as a solution to save someone some time in the future:
Under Junos on newer platforms, you'll see weird things like this:
me@switch> show system core-dumps
localre:
--------------------------------------------------------------------------
/var/tmp/*core*: No such file or directory
Host Crash Info:
--------------------------------------------------------------------------
-rw-r--r-- 1 root root 6737059 Apr 24 02:21 /var/tmp/corefiles/local-node.dcpfe.7892.1587694879.core.tgz
-rw-r--r-- 1 root root 6791284 Apr 27 02:07 /var/tmp/corefiles/local-node.dcpfe.7972.1587953252.core.tgz
-rw-r--r-- 1 root root 6668962 May 11 13:31 /var/tmp/corefiles/localhost.dcpfe.7914.1589203878.core.tgzfpc1:
--------------------------------------------------------------------------
/var/tmp/*core*: No such file or directory
Host Crash Info:
------------------------------------------------------
No corefiles present on the host.{master:0}
me@switch> file list /var/tmp/corefiles/
/var/tmp/corefiles/: No such file or directory
This is because the newer model switches now stores core files on the Hypervisor, not the Guest Junos image that you connect to.
In order to download core files (to attach to a JTAC case), you'll need to do the following:
start shell user root
vhclient -s
You'll now be in the Hypervisor as show by the prompt:
root@switch-node:~#
Now from here, you'll be able to see the /var/tmp/corefiles directory and be able to SCP files from the Hypervisor/Host machine to the Junos Guest:
scp /var/tmp/corefiles/*.core.tgz user@192.168.1.2:/var/tmp
The 192.168.1.2 IP is a built-in IP of the Junos Guest on most of the virtualised platforms - it's attached to the em2 inteface, which is hidden from your routing-table, but shows up when you run
show interfaces routing
Hopefully in the future the Junos command
request vmhost copy
will be implemented on the EX4300-48MP as it is on the MX10Ks so that you can copy files off the Hypervisor from within Junos, but for now (18.4R2S2) the above will work.