This will occur on Windows 2000 if you have never issued the diskperf -yv command to enable the Logical Disk measurements. When diskperf is not active, the corresponding Counters in System Monitor are zero. In Windows 2000, only the Physical Disk counters are enabled by default (this is equivalent to issuing the diskperf -yd command.)
In Windows NT, neither Logical or Physical Disk counters are enabled by default. To enable both sets of Disk counters, issue the diskperf -y command in NT 4.0. You must reboot the system in both Windows 2000 and NT 4.0 in order to activate the new diskperf settings.
Both Logical and Physical disk performance statistics are turned on by default beginning with Windows Server 2003.