56f1136b_Hadoop_Troubleshooting__Possible_Problem
[Symptom] Your DataNodes won't start, and you see something like this in logs/*datanode*: Incompatible namespaceIDs in /tmp/hadoop-ross/dfs/data [Possible Solution] You need to do something like this: bin/stop-all.sh rm -Rf /tmp/hadoop-your-username/* bin/hadoop namenode -format Be VERY careful with rm -Rf [Possible Problem] 
Your Hadoop namespaceID became corrupted. Unfortunately the easiest thing to do reformat the HDFS.
Hadoop Troubleshooting
Possible Problem