I'm no sure what part of <rsync from external drive to network drive> leads to filling up the internal system drive, but there's really only one reaction possible:
(╯°□°)╯︵ ┻━┻
The network drive became unmounted, but because Unix, that meant I just copied my backups onto my system drive until it was full.
a Schelling point for those who seek one
The network drive became unmounted, but because Unix, that meant I just copied my backups onto my system drive until it was full.