When I do
sudo umount /media/KINGSTON
umount: /media/KINGSTON: device is busy.
fuser -c /media/KINGSTON
I found that the process is thunar--deamon. I closed all file manager windows. When I try to umount it by right cliking on the desktop, it also gave an error :
An application is preventing the volume "KINGSTON" from being unmounted
I am affraid that killing Thunar is going to impact other process running in other terminal. How can I tell it that I don't need this usb key anymore so it stop using it?