'a few gigabytes' ... that means 10 or under? Sufficient ... remains to be seen.
df -h
moodledata/temp/backup/ is the build area moodle uses to create .mbz (backup) files.
zombie files ... there is a command line only utility for moodle called 'moosh'. It has some file commands that might help.
https://moodle.org/plugins/view.php?id=522
https://moosh-online.com/commands/
'SoS', Ken