Originally Posted by peterb 
Well I wouldn't say total safety, you still have a single poiunt of failure in the controller card, and if another error mangles the data on the disk, it is mangled on both. RAID is not really a substitute for backup, although syncing a raid solution to either another RAID or even a single drive is effectively a backup - provided it is kept offline once the backup has been taken.
OK, fair enough - I got a bit over-excited (so much pride in my two NASes ;-)) - BUT: my plan is to do exactly what you recommend: sync it to another RAID configuration. And I'm gonna do it even one step safer: I want to sync my two NASes (3TB total) to a new 5-bay 4.5TB total NAS that resides in a different country (my parents place :-)) - I'll do the first sync when they're on the same network and then ship the 5-bay home and do incremental syns from then on.
One question about that (tell me if I should open a new thread for this): QNAP has a nice sync feature built in that syncs to another QNAP via the www. What are you guys using to sync your NASes?