Thanks for the reply. Today the process completed, I think the total duration was about ~40 hrs (I cannot check the exact amount due to timezone mismatches). Yeah, it was plugged to USB3 slot (in front).
How can you do both rsync&mirror? I've following options to select. And I selected the last one.
Then a stupid question - does the rsync ever touch the SOURCE files (the files that are on my NAS) or always only the TARGET files (the USB disk). So, if I edit the files in USB disk rsync has copied (which I should never do), would it make those changes to same files present on the NAS when I do the next backup?