i recently bought wd mybook live. it's supposedly superior to dns-323 in many ways (better debian, faster etc.). it's web ui is really dumbed down though and there is no access to network speed and mtu settings (so no jumbo frame settings), default mtu is 1500. but i get upload speeds 37-38+ MB (megabytes)/s and ~10 GB file uploads in three minutes.
download still sucks. so it's definitely mac/osx thing and has nothing much to do with dns-323, it's drives or jumbo frames, at least for me..