It's possible to customize various attributes of the to-be-mounted filesystem by specifying mount options. For example, you can mount a filesystem as "read-only" by using the "ro" option:

# mount /dev/hdc6 /mnt -o ro

With /dev/hdc6 mounted read-only, no files can be modified in /mnt -- only read. If your filesystem is already mounted "read/write" and you want to switch it to read-only mode, you can use the "remount" option to avoid having to unmount and remount the filesystem again:

# mount /mnt -o remount,ro

Notice that we didn't need to specify the partition block device because the filesystem is already mounted and mount knows that /mnt is associated with /dev/hdc6. To make the filesystem writeable again, we can remount it as read-write:

# mount /mnt -o remount,rw

Note that these remount commands will not complete successfully if any process has opened any files or directories in /mnt. To familiarize yourself with all the mount options available under Linux, type man mount.