I'm a french user to and got the same problem...
I solved it by copying the files using windows instead my mac... and then it works!!!
I had a similar problem this morning: when copying the fun_plug files the Finder popped up a message (which I didn't care to read at first) and then files showed up in the DNS-320.
Looking closely they were there but had 0 Kb. The Finder error is error -36
The DNS-320 samba has a veto on the .name files (among others) for OS X and the the error comes up due to that. Easy to circumvent using the terminal to copy the files using cp or editing the smb conf.
No need to go to windoze.
BTW a DNS-321 works fine, so it is definitely the veto files in samba.
Bonne courage!