I use Ubuntu 17.04, though this problem was persistent in many previous versions. Long ago (a year or two, I think) I started to spot it.
Immediately (no more than a couple of minutes) after the system start up the system load indicator is starting to report writing to the disk, as it is shown on the next screen shot (sorry for a photo, since I don't know how to make a screenshot with the dropdown window visible)
As you can see, the disk write is being reported as 1.5 MB/s (though it is often 1-2 GB/s) and the iotop program reports no writing at all. Then, after five or ten minutes everything turns back to normal.
I tried to search for this, but advices which were given in this question seem to yield no result in my case, like the advice to use iotop.
Does anyone know what is going on here and should I be worried? Thank you.