You might want to try the various suggestions in the FAQ on FTP in Opus if you want to try to get Opus talking to the device via FTP.
Depending on the type of NAS (or whatever it is), there may also be options for how it is accessed. e.g. You may be able to configure it to appear as a normal network drive instead of an FTP site.
Check the device's firmware as well, as we've seen problems with other Buffalo devices that were fixed by firmware updates.