Why does mouse and cursor movement become erratic when I copy an entire memory stick to another stick?

I copied a 16 GB USB memory stick to another 16 GB stick (cloned it), by using the following command: cp /dev/sdb /dev/sdd It took about 27 minutes, and the cloning works. But during the entire copy process, the mouse and cursor movement become erratic, and lag terribly. This makes using the laptop during the copy almost impossible. This happened to me on a previous occasion, so the effect is reproduci ble. What causes this effect? How do I stop the erratic behaviour? Are you able to replicate the behaviour on your computer?