You probably know by now that you shouldn't just pull-out a USB key stick out of an Ubuntu computer. This can cause data loss. Instead, you must right-click the desktop icon and select Unmount Volume. However, sometimes you might see an error message along the lines of "An application is preventing the volume from being unmounted". If you have no applications open, this can seem confusing.
The error message isn't just referring to applications. Any Nautilus window that's currently browsing the memory stick will have to be closed, and if you're browsing the USB memory stick contents from the terminal then you'll need to cd away from that particular folder (ie cd to return to your /home folder). Then try again to unmount the key stick.
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