What performance counter tracks the percentage of time the hard drive is in use?

The performance counter that tracks the percentage of time the hard drive is in use is called "Disk Time" or "Disk Utilization." This counter indicates the amount of time the disk is busy processing read and write requests. It is typically expressed as a percentage of total time, helping to assess how effectively the disk is being utilized and whether it may be a bottleneck in system performance. High values may indicate that the disk is a limiting factor for performance.