As a previous poster indicated, the fun-plug does provide this support. I have used it to connect a 1TB USB drive to the DNS-323 (which is 320GB) and have the DNS configured to backup the entire DNS-323 to the USB drive every two hours (daily cycle), daily (weekly cycle) and monthly (annual cycle).
This works because the USB drive is EXT2 formatted and I'm using rsync to backup (using hardlinks to avoid making duplicate copies of files). From my XP box the USB drive looks to be several TB in size... There's examples of these scripts around, and it was fairly easy to set up.
I have been delighted with my DNS-323 and the guys at DLINK are to be commended for their open attitude towards their hardware.