Add a warning about gmirror truncating the last sector, which may, in

some cases, cause data loss.

PR:		128804
Reviewed by:	keramida, pgj, rene (as part of a larger change set)

en_US.ISO8859-1/books/handbook/geom/chapter.sgml

@@ -264,6 +264,13 @@ Done.</screen>
 	<filename class="devicefile">/dev/mirror/gm</filename> device
 	using the following command:</para>
 
+      <warning>
+	<para>Creating a mirror out of the boot drive may result in
+	  data loss if any data has been stored on the last sector of
+	  the disk.  This risk is reduced if creating the mirror is
+	  done promptly after a fresh install of &os;.</para>
+      </warning>
+
       <screen>&prompt.root; <userinput>gmirror label -vb round-robin gm0 /dev/da0</userinput></screen>
 
       <para>The system should respond with:</para>