In some situations after installing SNMP Helper Freeware or Pro Edition on a machine you may not be able to see all disk related sensors (especially on Windows 2000).
On the machine that - when queried - does not show all disk data in the SNMP Helper sensorlist please perform the following steps:
1) Open an OS prompt
2) Type "diskperf -y" (omit the "")
3) Close the OS prompt
4) reboot the server
5) try again in PRTG
This enabled the disk counters if they are disabled.
Note: If your diskdrives are fault-tolerant you may need to use "diskperf -y -e"
More info:
support.microsoft.com/kb/253251/EN-US/
www.microsoft.com/WINDOWS2000/en/advanced/help/diskperf.htm
www.microsoft.com/resources/documentation/windowsnt/4/workstation/reskit/en-us/06disk1.mspx