rsync - Sep 2005 - failed sync. Broken pipe

My rsync was working from a cron job for a long time. Suddenly, I
started to get the error:

rsync: writefd_unbuffered failed to write 85 bytes: phase "unknown"
[receiver]: Broken pipe (32)
rsync error: error in rsync protocol data stream (code 12) at
/SourceCache/rsync/rsync-20/rsync/io.c(909)

I am syncing to an external hard drive. It appears to be mounted properly.