Hi,
On the box on which nagios is running you need to install nrpe plugin.
On the box on which you want to monitor the disk, you need to install nrpe plugin. daemon and daemon-config.For more detail follow NPRE documentation in www.nagios.org
NPRE is configured in both client and server. I can monitor check_users, check_load on those linux host from nagios server. I just would like to know how to monitor over hard drives. Do you have service example ?