1. First check which is the directory configured for dfs data storage.
Login to Ambari UI -> Services-> HDFS -> Configs -> [search for dfs.datanode.data.dir]
Capture this list of directories defined here.
EG. i have list as below -
2. Login to the datanodes and go to the mount.
In my case- $cd /data01/hadoop/hdfs/
3. Check if there is any directory/data inside "/data01/hadoop/hdfs/" or "/data01/"
4. Other data than "**data" is consider as nondfs and which will show you in dfsadmin -report.
5. You need to get rid of those data which will lesser your NON DFS used.
You can share your output if you have any confusion/need help.