Wd My Book Duo in RAID 1 mode (2*4TB) works fine for me. I changed mode to RAID1 on Windows driven PC, formatted it in ext4 mode with Paragon Hard disk Manager on same PC, than attached to my home server, driven by Ubuntu 14.04 (Intense PC: IPC2 ) to USB3 port. After usual operations (sudo blkid to determine UID and editing /etc/fstab it works fine for me. Important: when I opened drive in Paragon Disk Manager after switching to RAID1 mode, there were 2 partitions: one small hidden NTFS formatted partition (it contains WD soft) and one large NTFS formatted partition. I destroyed both (on my mind this is must when you are intend to use drive with Linux) and created one partition on all drive volume, after this I formatted it to ext4.
One issue: after I connected drive to Linux machine, blue LED’s are always off. But drive itself works perfectly.