Windows – Disk Activity? What does it mean

I’m pretty sure it is the same as the Perfmon counter "PhysicalDisk _Total %time". (It tracks this counter in lock-step.)

Perfmon gives this description of that counter:

% Disk Time is the percentage of elapsed time that the selected disk drive was busy servicing read or write requests.

Basically, it is the percentage of elapsed time that the physical drive(s) were busy. So Even though your MB/s usage may be low, the % busy time can be high if the disk is heavily fragmented, or is otherwise thrashing on lots of small operations.

On a server where you would try to match the right level of high-performing disks to the workload, you would consider the disk subsystem to be a potential bottleneck if the % active (busy) time plateau-ed above 80%. On a workstation if this number is high, it just tells you that your disk performance is maybe what is slowing down your work.

As to your last question: I'm quite certain it is the physical disk and not any kernel or process time. While waiting on the physical diskthe kernel blocks whatever processes is waiting on that disk operation, and uses the CPU for whatever else needs doing.

If you're using multiple resources on the same hard drive, then this is normal behaviour. Hard drives are big, mechanical devices which are good at one thing - only doing one thing at a time. You can only read/write to a single sector at a time, so attempts to use the hard drive simultaneously usually result in thrashing. This isn't a side effect of anything other than the hardware working as designed.

If you're doing multiple things on a mechanical hard drive at once, you might find better results by performing these actions one at a time. If you're talking about file transfers, considering replacing the Explorer file copy handler with another program which supports transfer queuing (such as TeraCopy, which will also allow you to pause the transfer if you need fast disk access temporarily).

You can also help to mitigate these effects with the use of a solid-state drive, but it is no guarantee - it only helps because of the drastically reduced random access time. You can still have thrashing with an SSD, and an SSD still has the same limitation - it can only read or write to a single sector at a time.

If an SSD isn't the route you want to go, consider using a RAM disk, performing any data processing in-place in memory, add additional drives (not partitions) for each task, or performing I/O intensive tasks on non-OS drives.