Why does my desktop lock up when I copy lots of files to an USB drive?

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http://unix.stackexchange.com – My desktop is usually very responsive, even under heavy load. But when I copy files to an USB drive, it always locks up after some time. By "lock up", I mean: Moving focus from one window to another can take 10-20s Switching desktops can take 10-20s Videos don't update anymore (in YouTube, the audio continues to play, only the video freezes) The system load isn't exceptionally high when this happens. Sometimes, I see a lot of white on xosview indicating that the kernel is busy somewhere. At first glance, it looks as if copying files to the USB drive would interfere somehow with compiz but (HowTos)