Sometime when you umount a device you can have this error:
$ umount /mnt/target umount: it seems /mnt/target is mounted multiple times
and you obtain this message when using root privileges
$ sudo umount /mnt/target umount: /mnt/target: device is busy. (In some cases useful info about processes that use the device is found by lsof(8) or fuser(1))
to check the problem you can use mount or mount -v
$ mount .... /media/VM/QEMU/Lnx8.img on /mnt/target type ext4 (rw) /sys on /mnt/target/sys type none (rw,bind) /dev on /mnt/target/dev type none (rw,bind) /proc on /mnt/target/proc type none (rw,bind)
In this case there are many mounted points so it’s necessary umount them before umount /mnt/target
sudo umount /mnt/target/dev sudo umount /mnt/target/sys sudo umount /mnt/target/proc
If this can not resolve the trouble, you can use fuser to show pids of the processes which using mounted device
fuser -m /mnt/target
Then you have to kill thoose processes and then try again umount
sudo kill id-process or sudo kill -9 id-process
You can use lsof, too. Lsof tell you open files.
Sometime it is useful umount with attribute -l
umount -l /mnt/target
As ultimate choice we have to restart the computer this removes all mounted points.