Head on over to .mac and pick up your copy. According to the site:
Backup 2 Public Beta lets you select any folder on any hard drive as a destination for your backup—use external FireWire hard drives, network servers, or even an iPod. You can also schedule these backups for an extra measure of protection.
Backup 2 Public Beta expands CD and DVD support beyond Apple-supplied internal optical drives to many third-party drives. This version even lets you mix different optical disk types in a single backup set.
When you use the Back up to Drive mode, you need to click Set… to pick a destination for the backup. You will be selecting a name for your backup and the location where it will be saved, such as on an external FireWire hard drive.