Is there a way on a RHEL server to blacklist external hard drives from SNMP configuration so that if and external HD fills up, SNMP doesn't 'detect' it.
This depends on your SNMP daemon (and which MIB you're using).
If you're using net-snmp and the Host Resources MIB: No.
If you're using net-snmp and the
If you're using some other SNMP daemon: You're on your own :-)