The umount command detaches the file system(s) mentioned from the file hierarchy. A file system is specified by giving the directory where it has been mounted. Giving the special device on which the file system lives may also work, but is obsolete, mainly because it will fail in case this device was mounted on more than one directory. Note that a file system cannot be unmounted when it is `busy‘ – for example, when there are open files on it, or when some process has its working directory there, or when a swap file on it is in use.
# umount /testfolder umount: I/O error umount: cannot unmount /testfolder
If you encountered above error while mounting a file system, you can force the system to mount the file system by running below commands.
# umount -f /testfolder
The command will force unmount in case of an unreachable NFS system.
or, you can try this command,
# umount -l /testfolder
The command will is named as lazy unmount. It will detach the filesystem from the filesystem hierarchy now, and cleanup all references to the filesystem as soon as it is not busy anymore.