Ideally, you should invest in a physically separate storage device setup in at a physically different location. You can perform scheduled remote backups (via the Internet) to make the backup process relatively painless (and more reliable). Depending on the quantity of data you have, it may be best to perform the first backup locally, and schedule incremental backups remotely from that point forward.
For an added layer of protection, you can still purchase a relatively inexpensive external drive (for another local backup) to supplement your DNS-323.