@derobert I think I've successfully silenced smartd. I've added the parameters -v 197,increasing -v 198,increasing to my DEVICESCAN line in /etc/smartd.conf:
DEVICESCAN -d removable -n standby -m root -v 197,increasing -v 198,increasing -M exec /usr/share/smartmontools/smartd-runner -s L/../02/./17