Documentation
smart_ plugin
http://munin-monitoring.org/wiki/PluginPortrait/smart_
Overview
See the list of hard drives in your system.
Notice entries like /dev/sda, /dev/sdb etc. Ignore the partitions labelled /dev/sda1, /dev/sda2, etc.
# ls /dev/sd*
Configuration
Check where the munin plugins are storted in your system. Depending on your distribution, the plugins might be found here:
usr/share/munin/plugins/
On Gentoo, they are found here:
/usr/libexec/munin/plugins/