-u The -u flag indicates that the status of an already mounted file
system should be changed. Any of the options discussed above
(the -o option) may be changed; also a file system can be changed
from read-only to read-write or vice versa. An attempt to change
from read-write to read-only will fail if any files on the file
system are currently open for writing unless the -f flag is also
specified. The set of options is determined by applying the
options specified in the argument to -o and finally applying the
-r or -w option.