Manual will have most answers to your questions. Please read it.
my $.02, even if you get the Duo, still need to back it up. A Duo is redundancy, not backup. Make sure you understand the difference.
Here are some topics on it:
http://community.wd.com/t5/My-Book-Live/Safepoints/m-p/560030/highlight/true#M21040
You can also use rsync, since these devices have Debian Lenny.
http://community.wd.com/t5/My-Book-Live/rsync-backup-w-o-Safepoint/m-p/593049/highlight/true#M24971
