B&H has the WD 8TB My Book Thunderbolt Duo for $479 (reg. $549) through Jan. 27. Check it out here at B&H Photo.
These Thunderbolt Duo drives can be configured in RAID 1 but the big payoff is RAID 0, which provides speeds up to 300MB/s. You can also read my first impressions of the WD My Book Thunderbolt Duo drives with real world speed tests.