To Enable Append operations on Hadoop

    Open the file on the path  ${HADOOP_HOME}/conf/hdfs-site.xml
    copy the following code into the file between the <configuration> tag

  <property>
       <name>dfs.support.append</name>
       <value>true</value>
       <description>Does HDFS allow appends to files?
          This is currently set to false because there are bugs in the
          "append code" and is not supported in any prodction cluster.
       </description>
  </property>

[Ref: http://hadoop4mapreduce.blogspot.in/2012/08/two-methods-to-append-content-to-file.html ]