After a few days of usage I noticed the same behavior as the op states.
To explain what happens, here is the original screenshot that Nanor posted:
View attachment 11633
As you can see, a few programs use just a fraction of the HDD but the disk
activity is monitored.
If you look at the transfer rates, just about 0.5mb spread to 5 programs is enough to produce a 100% activity graph.
This means that the disk is used but not that a program uses it all.
We didn't had this function on Win7 or earlier but the 100% usage came very frequently (you could hear the spinning disks doing something).
What all this means is that as long as the disk is active, even for 0.01kb transfers, there will be a noticeable activity of the disk.
When the disk is not used AT ALL, only then we can speak of 0% reports.
I have a pretty clean, offline Win8 pc and had 99% disk activity for 2 processes with 0.1kb transfer.
Look at the details in the performance tab, and right click on the disk graph: you'll see that the activity is displayed, not the transfers. Look at the transfers and from 10 up to 100mb/s (some 100% transfer, the max that the disk can give) per process (memory dump from system took 78mb/s here!) usage we can speak of serious issues or data transfers.
There are no background executables that are not seen because Task Manager in Win8 runs as administrator (elevated) by default.
Sorry I had to find this out myself:
go to details tab and choose columns, you can choose "Elevated" and you can see there which program runs as admin (marked with yes).
The mentioned Process Monitor program seems useful but is not always necessary. If you got a clean stable pc and see 100% activity of disk, look at the transfer rate and usually it's nothing. So if more than 1 process is using the disk even with very low transfers, this can result in 100% activity.
A long period of 100% activity means that a program or more use the disk constantly or periodically or all at the same time, or just on or two at a time. This can be your browser viewing or you listening to something, checking mails... The pagefile of the system can also use the disk (as I said very low transfer speeds are enough).
As long as the HDD (heads are moving) or SSD transfer some data, the 100% activity can be achieved easily so it seems.
This is NORMAL and we don't have to worry. We'll just have to get used and understand the new Task Manager graphs.
It's normal for the disk to be used. System is running well.
If you're still worried and pc might no be as clean as you might want, use Process Monitor as described in a previous post.
I'm convinced after seeing the graphs myself:
100% activity is not 100% transfer.