Problem with pg_baselinebackup

Hi Folks,

I'm pretty new to Postgres. I have an extensive Oracle and MySQL background here.
We are putting up test and production servers.
I'm interested in doing online backups with WAL archiving turned on so that I can do point in time recovery.

My database files are stored in /u01/postgres/data and /u02/postgres/data/confluence_data directories.
My WAL files are in /u03/postgres/data/pg_log

I want to backup my database to the directory /u03/postgres_backups/pg_basebackups/"directoryname_date_timestamp"

I'm trying a command like this:
$ pg_basebackup -x --pgdata=/u03/postgres_backups/pg_basebackups/pg_basebackup_16-Jan-2014:10:15:00 --label=TEST

After entering the password, I'm receiving the following error:
pg_basebackup: directory "/u02/postgres/data/confluence_data" exists but is not empty

The directory that it is complaining about ("/u02/postgres/data/confluence_data" is a database directory with database data in it. Not the directory where I want the backup files to go. I expected that the switch
--pgdata=/u03/postgres_backups/pg_basebackups/pg_basebackup_16-Jan-2014:10:15:00

Would send the backup to that directory. Can anyone enlighten me on why it would complain about a directory where database files reside?