My recommendation to you is to buy two HDDs to fill the first two slots of the DNS-343 and schedule a data transfer from the DNS-323 to the DNS-343.
I had to perform the same exercise when I purchased my second DNS-343, except I had to push nearly 4 GB of data from my primary DNS-343 to my backup DNS-343. In any event, it's always a good idea to keep a backup of your critical data on a separate physical NAS, and you can use the DNS-323 for that purpose.