Does anyone know where does the back up on the server directadmin keep back up on mount drive back up. I'm not sure where is this location, if anyone know just let me know.
that SHOULD be telling it to back up in the root folder (/) under the backup directory. (/backup/)
An easier way of doing that would be to just configure it within the admin panel under "Backup Path". Put a full path there. Example: /home/admin/backup and have the backup folder under that directory be mounted to whatever drive you would like it to backup to.
I'm not positive and I'm not looking but I believe $MOUNT_POINT points to /home. You can check in the sysbk config file. But as evil_smurf suggests, I always use a full hardcoded path.