I can't answer your question about testing for data integrity, that would be internal to Dlink. But I can tell you that I very much agree with GRJ on his response to you. For myself, I will never trust a single device to be my sole backup. I work in a data center and even there, with multiple copies and vast RAID arrays, we still keep a copy of everything on either tape or optical platter. At home I have my 321 set to RAID 1, but I also have a different brand of drive, identical size and internal to my desktop PC that does an incremental backup of the NAS box every day while I sleep. And I also do a small FTP backup of critical data to a friend's NAS box about 1200 miles from my home.
If something like the controller board fails in the NAS during a write or a cache dump, chances are that you will lose both drives. The NAS can help when trying to prevent against drive failures, but it can't protect against NAS failures. So if you have something you can't do without or can't stand to lose, do a backup to a separate machine, preferably in a separate province or state.
Just an opinion, from someone who has been in the IT field for well over 20 years now.
Ryder