HDFS fsck only checks the files that are persisted on hdfs and not open files. Since you're seeing just one missing block in the UI warnings of CM and NN and no missing blocks in fsck output, this would indicate that the missing block alert is being generated from a file that is open in the memory and is most likely a false alarm.   This should go away when the NN role is restarted or the cluster is restarted, probably during your next maintainance window. ... View more