I have looked at several different system that had disk usage near 100% Problem is that each one ended up having a different cause so it is hard to say what the fix would be. But I can give you some ideas to help isolate the cause down.
You start task manager, select the performance tab, open resource monitor, select disk tab
then open a cmd.exe as a admin (windows key+x+a)
now you stop various suspected services and watch how the system responds in resouce monitor.
for example:
net.exe stop superfetch (hit enter and see if your disk usage goes down after a min or two)
net.exe stop "Windows search"
I mention these two because I have seen disk corruptions that made these two services go nuts and take 100% disk bandwidth
I have also seen tasks in the task scheduler that did the same thing until you killed the task.
in all the cases you had to find out the cause of the problem and fix it and the service then worked as expected.
for example, one case had a filesystem corruption that made a loop in the directory structure such that when windows or any program scanning the files would never end and just continue scanning the same set of files forever. In one case it was building a search index and the database just kept growing and growing in size until all of the hard drive was used.
in most cases all you need to do is run cmd.exe as a admin then
chkdsk.exe /f /r on your drives and the condition that causes the problem gets fixed
there are also conditions where windows 8.1 will really use your disk while it is attempting to relocate data off of bad spots on your disk drives.
older versions of windows did not do this. windows 8 will attempt to read data from your disk drive even if you have not requested it. It is looking for errors during the read process so it can locate bad sectors before your data is lost. It will read, if it gets a read error, it will read the spot over and over and try to get a clean copy of the data, it will then move the data to a new good spot on the drive and mark the old spot as bad.
This process can take days on some drives and your system will seem to be at 100% disk used for no apparent reason. It will take much longer on a laptop it the laptop is allowed to sleep. This process is needed because a lot of OEMs do not format and install the os, they just plop a entire OS image on a drive with the assumption that all sectors of the drive are ok.
(I guess that is another point, turn your system to high performance and let it run at idle overnight and see if the disk is at 100% usage in the morning)
- there are also software bugs that will peg your disk access at 100 % but these almost always end up being bugs in 3rd party drivers. It is not because microsoft makes great drivers, it is because microsoft has windows update and when a microsoft driver breaks they get 100,000 automatic bug reports and then they put a fix in the windows update and you download the update and don't ever hit the problem. When the bug is in a 3rd party driver microsoft still gets the 100,000 bug reports but they tell the vendor of the driver, that vendor makes a fix and puts it on their website. Now you have to hit the bug, figure out the problem is caused by the vendor, look at the vendors website and hope they put the fix there. Not a fun process. After years of this problems and updates I find that about 95% of the problems are in 3rd party drivers, and you just have to figure out which one.
sorry if this is not so helpful, it is hard to figure out some problems when you can not actually get your hands on the system.