Reduced pause between retries to one minute.
Repeated the back-port. On closer inspection, more sync() needed.

It seems mapping fsync() to sync() is ok (if less robust under serious
system failures. However, mapping the newer hflush() to flush() lost 
the meaning, which is actually best mapped to a sync(). 

See http://blog.cloudera.com/blog/2009/07/file-appends-in-hdfs/
Now working again.