How are your volumes set up?
I find it strange that you have not reported the problem before now. Have you removed the drives - noting which slot they were in - and tested them using the WD disk test tools?
Depending on how you setup the NAS you should be able to keep your data safe - two RAID 1 sets with the second acting as a backup to the first plus a backup to a removable 3TB drive should do what you are looking for. This, of course restricts you to a maximum capacity of 3TB.
The DNS-345 we have under test (considering it as a replacement for the old DNS-343) is set up as above and we have not experienced any problems with it. So much so, we are going to add it to our list of devices that we recommend to our clients.
Since your unit is still under warranty have you contacted your supplier with your problems?
One last thing. If you demand absolute reliability and do not have backups then are you prepared to spend something in the region of $10,000 to $15,000 per year for a managed service supplier to take over the job of looking after your data because paying the odd hundred more for another NAS isn't going to cut it.