It seems that the Net-SNMP snmpd daemon doesn’t always recognize a systems drives. I recently encountered this on some RHEL6 servers.
The solution is simple, add the follow option to the snmpd.conf file and restart snmpd.

includeAllDisks 1

This forces snmpd to monitor all the systems drives.
If you have NFS mounts, you may want to tell snmpd to ignore them with the following option.

skipNFSInHostResources 1

Another option is the disk option that allows you to specify specific drives instead on tracking everything.
You can read up on all these options and more in the snmpd.conf manpage.  The most recent version is available online at http://www.net-snmp.org/docs/man/snmpd.conf.html.