56f1136b_Hadoop_Troubleshooting__Possible_Problem
[Symptom] Your DataNode is started and you can create directories with bin/hadoop dfs -mkdir, but you get an error message when you try to put files into the HDFS (e.g., when you run a command like bin/hadoop dfs -put). [Possible Solution] Go to the HDFS info web page (open your web browser and go to http://namenode:dfs_info_port where namenode is the hostname of your NameNode and dfs_info_port is the port you chose dfs.info.port; if followed the QuickStart on your personal computer then this URL will be http://localhost:50070). Once at that page click on the number where it tells you how many DataNodes you have to look at a list of the DataNodes in your cluster. If it says you have used 100% of your space, then you need to free up room on local disk(s) of the DataNode(s). If you are on Windows then this number will not be accurate (there is some kind of bug either in Cygwin's df.exe or in Windows). Just free up some more space and you should be okay. On one Windows machine we tried the disk had 1GB free but Hadoop reported that it was 100% full. Then we freed up another 1GB and then it said that the disk was 99.15% full and started writing data into the HDFS again. We encountered this bug on Windows XP SP2. [Possible Problem] 
Creating directories is only a function of the NameNode, so your DataNode is not exercised until you actually want to put some bytes into a file. If you are sure that the DataNode is started, then it could be that your DataNodes are out of disk space.
Hadoop Troubleshooting
Possible Problem