Source: https://unix.stackexchange.com/a/111407/77354
-d ignore is a new directive which allows ignoring a device from DEVICESCAN.
If we want to ignore the disk /dev/sdb, edit /etc/smartd.conf and add /dev/sdb -d ignore before the line that starts with DEVICESCAN
It should look something like this:
...
# The word DEVICESCAN will cause any remaining lines in this
# configuration file to be ignored: it tells smartd to scan for all
# ATA and SCSI devices. DEVICESCAN may be followed by any of the
# Directives listed below, which will be applied to all devices that
# are found. Most users should comment out DEVICESCAN and explicitly
# list the devices that they wish to monitor.
/dev/sdb -d ignore
DEVICESCAN -d removable -n standby -m root -M exec /usr/share/smartmontools/smartd-runner
...I needed to stop the warning email messages from the smartd daemon, yeah I know my disk is failing so just shut up.
Also found out about the interesting -M diminishing directive
diminishing - send additional warning reminder emails, after a one-day interval, then a two-day interval, then a four-day interval, and so on for each type of disk problem detected. Each interval is twice as long as the previous interval.