I am having the same problem with my new DNS-325 from my iMac running OS-X 10.6.
As best I can tell at this point, it's related to some limit on how many files or how much data I can copy at once to the NAS drive. I'm still playing with this. I'm trying to restore all my iTunes music to my DNS-325 which I had backed-up on another USB drive. The iTunes Media folder has a Music sub-folder has many other sub-folders for each artist. As long as I limit the number of files I copy in one batch to less than around 920 files or 3.5Gb at a time, it seems to copy fine. It's just going to take a while to copy all my files. This is really annoying...
Does anyone know where this limit might be set, what it is, how to change it? I'm not sure if it's an OS-X limit or a DNS-325 limit. Anyone have any ideas?