how do I find out what is using close to the full disk?

I am glad to provide all this information. How do I find it?

Following is a summary of why you see what you see vs. transfer rate in which many posts attempt to explain what’s going on.
--- I know you want to know why there appears to be such a discrepancy but many posts try to explain why.
--- In my mind when a user notices Disk Usage showing 100% it means something is going on and the reason should be determined.

Post #1: You mention “Task Manager reports .26M being used: but disk tab shows 100%”.

Post #3: Kari mentions “The Disk column does not show how much storage space is used. It shows disk activity.
--- That’s a pretty good summary and is supported in future posts.

Post #9 you mention: “The value at the top says it is completely being used. But it's only .26.”
--- Subsequent posts explain why.

Post #12: Bree mentions “If you add up all the disk usage figures you get 2.9 MB/s - that's Megabytes per second. It's showing the rate at which data is being read or written to the disk, not the total size of the files on the disk. The 100% means that the disk is working at the maximum transfer rate it is capable of, not that you have used up all the disk space.”
--- The Bold sections are my doing.
--- That’s an excellent explanation of what’s going on.

Post #19 you mention
“When it gets to one hundred percent my system slows down”
“I want to know why the numbers at the bottom are different regardless of the fact that it says 100%.”
“I would expect the numbers to add up to the same thing when it's full, not only .26 megabytes sometimes. In other words, how can .26 megabytes be full”
--- The numbers will not add up because the disk activity is the issue vs. disk usage.
--- As bro67 reports in post #8:” Task Manager is not the way to look at real use.”
--- To me, the computer slows down because it’s extra busy running at the maximum transfer rate its capable of.

Post #25: dalchina asks “What type of disk do you have?”
--- Please what type of HDD: what size is it?

Post #26: bro67 mentions “Your system is slowing down if there are not enough available resources. In Task Manager, when it states that it is at 100% disk usage, it means that it is pushing so much data, that it is saturating your Disk I/O.
--- That is very true.

You might want to determine which process(es) are the causing the disk to operate at its maximum transfer rate.
I have found an article that seems to allow you to determine that and I have written what I suppose is a mini-tutorial. It’s based on Windows 8, but I adjusted it for Windows 10 and tested it on my Windows 10 and the process is pretty straight-forward. Since I don’t have any problems on my computer though I can’t tell if it will work when it applies to your situation. For my sake, I want to go back and review how I’m wording the steps to be followed. I’ll be glad to post my mini-tutorial version2 if you are interested.

I just went through this set of instructions twice to verify how to check my HDD usage on my Windows 10 laptop.
--- This is a step-by-step guide with some additional reporting information.
--- I’m not an expert and some interpretation will be needed depending on the situation at hand.
--- I do not know if it will always be helpful on everyone’s computer but I hope it will be helpful at least this time.
--- It uses Task Manager > Performance tab > Resource monitor > Disk tab.
--- Along with End Process in the Process tab when applicable.
--- If it doesn’t help find the troublesome process: well, at least I tried.

How to check your Hard Drive usage
How to Check Your Hard Drive Usage - Ophtek
NOTE: This article is for Windows 8: I am adjusting it for Windows 10 and it ran fine on my Windows 10 laptop
Paraphrase: If your computer is running sluggish, your HDD usage could be too high.
--- Here’s how to determine if it’s working too hard.
Programs malfunctioning in the background can read and write unnecessarily from your hard causing your computer to slow down.
Clicking on your hard drive will show the free space available. Even if you have plenty of space available, your hard drive could be reading and writing too much.


To find out if that is the case, open the Task Manager > Performance tab > Resource monitor > Disk tab.
Under Processes with Disk Activity, you can view which processes are accessing the hard drive for reading and writing. Any process that looks unfamiliar should be investigated. Ask here if you wish.


Under Disk Activity, you can view the exact bytes per second each process is using for reading and writing. If any process is using an inordinate amount reading and writing capacity, several possibilities exist.

One possibility is the software governing the wayward process could be malfunctioning. To find out if this is the case, close and re-open the program. If you cannot close the program normally, go back to the Task Manager.
Under the Processes tab, highlight the process in question and click End Process.
Example


Check the Resource Monitor again to see if the process is still reading and writing with the same bytes per second. If it is, you might want to uninstall and re-install the program.
--- Install the program again and see if the reading and writing speeds are different.
--- If the high reading and writing speeds persist, the computer could be infected with malware.
--- If malware is not the culprit for the aberrant reading and writing, your hard disk could have errors.

Right click your hard drive and select Properties > Tools tab >Error checking > Check.
You might see a message that reads “You don’t need to scan this drive”.
--- If you see that, don’t scan it unless you want to.
My comments: You may also check the Optimize and Defragment drive section.
--- You can do them if you wish but ask about them in our Windows 10 forum if you feel the need to do so.
If this doesn’t manage to sort out the reading and writing problems, you could have a defective hard disk.

Reporting Information section


Processes with Disk Activity reporting information
Image: Image process executable file name.
PID: Process ID.
Read B/sec: Average number of bytes per second read by the process in the last minute.
Total B/sec: Average number of bytes per second accessed by the process (read and write) in the last minute.


Disk activity reporting information
PID: Process ID.
File: Name of the file in use by the process.
Read B/sec: Average number of bytes per second read from the file in the last minute.
Write B/sec: Average number of bytes per second written to the file in the last minute.
Total B/sec: Average number of bytes per second read from and written to the file in the last minute.
I/O Priority: Priority of I/O transfers.
Response time (MS): Disk response time in milliseconds.


Storage reporting information.
Logical disk: Logical drive letter C:
Physical disk: Physical drive number.
Active time %: Percentage of time the disk is not idle.
Available space (MB): Amount of free space on the physical drive in MB.
Total space (MB): Total space on the physical drive in MB.
Disk Queue Length: Average disk queue length.