Some people like to mount partitions from other operating systems (DOS, Windows, OS/2, etc.) so they can use them under Linux. These partitions must be created and copied under their own operating system, and you should unmount them before copying your Linux partition. For example, if you have a DOS partition mounted at /dos, you must unmount it with this command:
Note that the command is umount, without the first letter n in the word unmount.
You should also unmount network drives.