If you are running these checks on Linux hosts, the default command in nrpe.cfg is not check_disk, but check_disk1. If these are Windows boxes, the default command is nt_check_disk_. Check your Nagios configuration and compare with the nrpe.cfg file. Also, I've seen nrpe.cfg files where, upon install, the lines containing the command definitions have been commented out, so double-check that, too.
I'm not sure about that other issue with the blank values for Disk Space, though.
Edited Tue Jan 17 2006, 06:22AM ]