Systemmonitor "all disks" rate doesn't include all disks. Is this a bug?

I’m currently copying data between 2 HDDs. In plasma-systemmonitor, I tried to graph the read and write rates of all disks but the write rate (at least) for “all disks” doesn’t include my destination HDD.

Screenshot shows:

  • All-disks read in blue
  • All-disks write in red
  • Write for my destination hdd in green

As you can see blue and green are up but red stays very low.