du -sh /hadoop/hdfs/data shows the space used and not the space available. You should check for the space available on the directory.
You can check the available space using df -h command.
# df -h /hadoop/hdfs/data
Also to know the available space for HDFS you can use hdfs command, which should show the available HDFS space :
#hdfs dfs -df -s /
And to add more space with one datanode you should either add space to underlaying filesystem where /hadoop/hdfs/data is mounted or create additional filesystem something like /hadoop/hdfs/data1 and configure datanode dir(dfs.datanode.data.dir) to have two directory paths in comma separated format.
You can also add HDFS space by adding another datanode to the cluster.