mysqldump --opt -u username -p password databasename > /home/httpd/vhosts/your domain/httpdocs/back.sql
seem not work use plesk Crontab
or use http://www.phpfreaks.com/script/view/11.php
Problems:
-There should be no space between the -p and the password. It should read as -pPASSWORD
Thank you,
FreePgs.com Admin