The Disk plug-in was unable to ascertain whether the partition is valid or not. This could more likely occur for NFS or remotely mounted disks.
The Disk plug-in documentation provides the following explanation.
In older Netprobe versions, the Disk plug-in uses the "df" command to detect if a partition is blocked. There were reported issues that "df" commands might not be cleaned up in rare circumstances. Hence, it is recommended to upgrade the Netprobe to version GA3.5.0 (PLI-2190). In the said version, plug-in performs data gathering in a separate thread and no longer calls the "df" command.