On NameNode web interface, you may get this warning:
WARNING : There are about 32 missing blocks. Please check the log or run fsck.
If the cluster was started less than 14 days before, it would be great to add: "Is dfs.data.dir defined?"
If at the point of that error message, that parameter could be checked, and error made "OMG dfs.data.dir isn't defined!" that'd be even better. As is, troubleshooting undefined parameters is a difficult proposition.
I suspect this is an easy fix.