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If you boot from an Ubuntu live CD, it will be possible to do 'anything' with the hard disk drive. You can unmount it, and nobody can
read/write. And you can mount it read-only, unmount it again and mount it read-write when you want to write. See the details in the manual page of mount
-o, --options opts
Options are specified with a -o flag followed by a comma separated string of options.
mount LABEL=mydisk -o noatime,nouser
For more details, see FILESYSTEM INDEPENDENT MOUNT OPTIONS and FILESYSTEM SPECIFIC MOUNT
Attempt to remount an already-mounted filesystem. This is commonly used to change the
mount flags for a filesystem, especially to make a readonly filesystem writable. It does
not change device or mount point.
The remount functionality follows the standard way how the mount command works with
options from fstab. It means the mount command doesn't read fstab (or mtab) only when a
device and dir are fully specified.
mount -o remount,rw /dev/foo /dir
After this call all old mount options are replaced and arbitrary stuff from fstab is
ignored, except the loop= option which is internally generated and maintained by the
mount -o remount,rw /dir
After this call mount reads fstab (or mtab) and merges these options with options from
command line ( -o ).
ro Mount the filesystem read-only.
rw Mount the filesystem read-write.