Change the directory for backup

This is driving me nuts.

I have tried different path but i can’t change the default one:
/Volumes/web/wordpress/wp-content/backupwordpress-bfd57c056e-backups

I have added following line to my wp-config.php:
define (‘HMBKP_PATH’ , ‘/Volumes/web/wpbackup’);

I’m running WP on a synology NAS, what is wrong?

ref.
Backups are stored on your own host server in /backups. You can change the directory by modifying the wp-config.php file (see FAQ – How do I modify my wp-config file?): define (‘HMBKP_PATH’ , ’your directory’); We do not have access to any of the backup files that have been created by BackUpWordPress.

http://ww.wp.xz.cn/plugins/backupwordpress/