Windows 10 Task Manager: "100% Disk"?

In Task Manager, it’s showing me “100% Disk”. What does a percentage mean when applied to Disk? CPU and memory utilization are obvious to me but what does this mean? It is related to I/O rates, not disk used. When it’s pegged like that, sometimes the highest ranking process is showing something like 4 MB/s. Then it goes back down to 16% and the highest process is like 16 MB/s. I cannot in any way understand the correlation between the overall % and the actual transfer rates.

Disk writing speed is not constant and subject to fluctuation due to:

  • physical location on the disk,
  • defragmentation of the disk,
  • the source of the data stream,
    and whatever program you have to scan the file like an antivirus software or other file managing software.

I think the OP is asking what exactly the number means. It’s obviously not the current transfer rate as a percentage of the drive’s max transfer rate - if it were, it would only show a high percentage when doing a sustained sequential read/write. But it seems to correctly show a large percentage when the drive’s performance is maxed out with random read/write.

I suspect it’s some complicated formula based on the command queue length (i.e. how many processes are waiting for the drive), disk active time (i.e. how much of the time the disk is idle vs busy), transfer rate, etc.


A percentage is a number divided by another number. Memory has a fixed physical quantity, and the percent is the amount actually allocated divided by the capacity. CPU has a capacity of x instructions per second and the percent is the amount of instructions actually being executed divided by that capacity, probably based on the most recent 1 second (that may be a bit oversimplified but that’s the idea).

For disk percentage, I have no idea what quantity they are measuring. It doesn’t seem to correlate to transfer volume.

It has nothing to do with the actual transfer speed, its a Disk-load rating.

The rating is for the resources Windows allocates to it - Memory bandwidth, i/o Bus allocation, host controller, super fetch and most importantly how many other programs, plugins, etc accessing your HDD at any given time - but again, it has nothing to do with the transfer speed as such.

Thanks. When CPU or Memory is high, I can review usage by individual processes to see where the hog is. But I occasionally see Disk at 100% and I can find no way to trace the source(s). How can you diagnose this?

Windows Resource Monitor. (perfmon.exe) > Performance > Monitoring Tools > (right-click) > Resource Monitor.

Alternatively: perfmon.exe /res

We seem to be going in circles here.

When my disk usage is very high, how do I tell which process(es) are responsible for it? Performance Monitor just shows resource usage at the system level, not the process level. Task Manager just shows transfer volumes.

If my disk is at 100%, I want to know which process is using 75% out of that 100%.

<never mind, mistaken>

Actually if you look at the task manager under “Performance,” the graph is clearly labeled as “Active time”, and if you hover, it explains that this graph shows the “Percentage of time the disk is processing read or write requests.”

I don’t think there’s a way to know how much time the disk is spending on each process. For one thing, hard drives don’t process commands one at a time. The commands aren’t even processed in the order they are received (NCQ). So I think it’s impossible to define how much time is spent responding to a particular process.

That can be enough in most cases. You can sort the processes by the various columns by clicking in the box with the column heading. It still only shows the processes by MB/Sec but you can see if it’s just the top one sucking down the majority or a couple that are all running heavy on disk IO. It’s not a percentage but rank ordering and the raw numbers is enough to see the process(es) at the root of the problem.

Ok, so you are having a problem with your Disk showing many times 100%, correct?

This hints at your PC is having some issues and there are many causes and fixed:

It’s stuff like in this Link

Or stuff like in this Link

Or just good old Spyware, Malware or a Virus.