Copying files freezes

[GTriderXC]

1. I wanted to copy a big amount of data between partitions on my USB external drive (internally).
2. I did a mistake and added a second, parallel copy process.
3. A added a delete process
4. Copying became slower and slower...
5. I touched with a mouse pointer an audio file so that it started to play in the background.

Since then Caja froze as well as the destop. I'm able to work in a terminal as well as Programs>Accessories>"Files" (I think it is Nautilus) that I'm showing in the photo as an attachment.

I bet it is a known bug but which process I can kill to make the system stable again without rebooting? The problem is that files still seem to be copied in the background. Actually I mv them not cp so it is now a pretty bad idea to abort the before it stops.

I use 16.04.2 64bit