Hm. I am afraid to recommend this because it is not officially supported by D-Link and requires the use of Linux command line tools. If you are not good at typing commands, reading manuals and such, or if you have really precious data on your box you might be better off ignoring this....
But if you are into some adventure, search for 'fun_plug' for the DNS family ( http://wiki.dns323.info/ ). Installing it is as simple as copying some files onto your first drive and restarting the DNS-321. But all it gives you is (by default) a command line access to the DNS-321. From there if you know how to use linux commands it is pretty simple. ('cp' or 'mv' the files and that's it.)
Again, if you are not familiar with Linux a little bit, you might be better off just copying the data off the box and back again.