The prerequisites to unmounting file systems are:
You must be superuser.
A file system must be available for unmounting. You cannot unmount a file system that is busy. A file system is considered busy if a user is accessing a directory in the file system, if a program has a file open in that file system, or if it is being shared. You can make a file system available for unmounting by:
Changing to a directory in a different file system.
Logging out of the system.
Using the fuser command to list all processes accessing the file system and to stop them if necessary. See "How to Stop All Processes Accessing a File System" for more details.
Notify users if you need to unmount a file system they are using.
Unsharing the file system