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Sometimes when I want to umount a device, e.g.

sudo umount /dev/loop0

I will get the message

umount: /mnt: device is busy.

        (In some cases useful info about processes that use

         the device is found by lsof(8) or fuser(1))

I usually solve this issue by closing a console window (in my case xfce4-terminal) and then umount.

What does this problem mean? Is there some smarter solution?

It means that some process has a working directory or an open file handle underneath the mount point. The best thing to do is to end the offending process, change its working directory or close the file handle before unmounting.

There is an alternative on Linux though. Using umount -l calls a "lazy" unmount. The filesystem will still be mounted but you won't be able to see or use it, except for processes that are already using it. When the offending program exits (through whatever means) the system will "finish" unmounting the filesystem.

Given your "usual solution", it means that the shell you have running in your console window has a directory in a file system on that device as its current working directory.

Linux, and Unixes in general, want very badly to keep a file system mounted if a process has a current working directory in that filesystem.

You could just use cd in the console window to get out of a directory in or under /mnt rather than killing the console window, and the shell running inside it.

You can also use fuser to kill all processes using the mounted file system.

fuser -cuk /mnt

Options:

-c     

    Same as -m option, used for POSIX compatibility.



-u, --user

    Append the user name of the process owner to each PID.



-k, --kill

    Kill  processes accessing the file. Unless changed with -SIGNAL, SIGKILL is sent. An fuser process

    never kills itself, but may kill other fuser processes. The  effective  user  ID  of  the  process

    executing fuser is set to its real user ID before attempting to kill.kill.



-m NAME, --mount NAME

    NAME specifies a file on a mounted file system or a block device that is  mounted.  All  processes

    accessing  files  on  that  file  system  are  listed.   If  a  directory file is specified, it is

    automatically changed to NAME/. to use any file system that might be mounted on that directory.

Check for yourself at explainshell.

I encountered the same problem today ,and I've succeed to solve it .
my problem is result from mounting a directory to my current working directory,so when I run "umount" ,it happened.
the way to solution it is change the current directory and then run "umount ..".
i hope it works.

I discovered a situation which can interfere with unmounting a device that I will share as an answer even though this question is very old.

If you have NFS shares being hosted on the machine, and any of those NFS shares are backed by the device you are trying to unmount, you will have to stop sharing them (best done by stopping the NFS service, e.g. sudo service nfs-kernel-server stop on recent versions of Ubuntu). The NFS server may not show up in lsof or fuser, which can make the issue difficult to identify.

For me, the "device busy" error came up when I was inside that folder and I was trying to umount it

// Based on @klapaucius comment