To balance loads on network servers, you need to know how busy the server hard disks are. You can determine this by using the PhysicalDisk\ % Disk Time counter, which indicates the percentage of time a drive is active.
If the results of PhysicalDisk\ % Disk Time is high (more than 90 percent), you can check the PhysicalDisk\ Current Disk Queue Length counter to see how many system requests are waiting for hard disk access. The waiting input/output (I/O) requests should be sustained at a number no more than 1.5 to 2 times the number of physical hard disk spindles.

Most hard disks have one spindle, although redundant array of independent disks (RAID) devices usually have more. A RAID device appears as one physical hard disk in System Monitor. RAID devices created through software appear as multiple drives (or instances). You can either monitor the physical hard disk counters for each physical drive (other than RAID), or you can click All Instances on the Add Counters dialog box to monitor data for all of the computer's drives.

Use the values of the PhysicalDisk\ Current Disk Queue Length and PhysicalDisk\ % Disk Time counters to detect bottlenecks in the hard disk subsystem. If the PhysicalDisk\ Current Disk Queue Length and PhysicalDisk\ % Disk Time values are consistently high, consider upgrading the hard disk or moving some files to an additional hard disk or server.

The system maps physical drives to logical drives using the same instance name. Therefore, if you have a dynamic volume that consists of multiple physical hard disks, instances might appear as Disk 0 C:, Disk 1 C:, and Disk 2 D:, (where C: is made up of physical drives 0 and 1). If you have two logical partitions on a hard disk, the instance appears as 0 C: D:.

For hardware-enabled stripe sets, statistics for each hard disk are not available. You can obtain this data only when monitoring stripe sets enabled in software. If you use a RAID device, the PhysicalDisk\ % Disk Time counter can indicate a value greater than 100 percent. If this happens, use the Avg. Disk Queue Length counter to determine the average number of system requests waiting for hard disk access.