I have a DLink DNS323 which is the same product you mentioned. The drives inside the DNS323 are formated in ext2/ext3 which is the linux standard file system. If its a single drive you should be able to plug it into your computer after you install an ext2 driver to recognize it.
if your looking for a Windows ext2/3 filesystem driver. You can also try linux live boot cdroms that should allow you to migrate the data off the drive.
If you had 2 drives in the unit then it gets complicated. You would have to figure out the raid configuration and recreate it to retrieve your data.
Hope this helps