Generally, all mounted filesystems are unmounted automatically by the system when it is rebooted or shut down. When a filesystem is unmounted, any cached filesystem data in memory is flushed to the disk.

However, it's also possible to unmount filesystems manually. Before a filesystem can be unmounted, you first need to ensure that there are no processes running that have open files on the filesystem in question. Then, use the umount command, specifying either the device name or mount point as an argument:

# umount /mnt

or

# umount /dev/hdc6

Once unmounted, any files in /mnt that were "covered" by the previously-mounted filesystem will now reappear.