BuyDig offers the Western Digital 4TB My Book Essential USB 3.0 External Hard Drive, model no. WDBACW0040HBK-NESN, for $159. Coupon code "HDD30" cuts it to $129. With free shipping, that's the lowest total price we could find for a 4TB My Book external hard drive by $21, although we saw it for $10 less in December. It features automatic, continuous backup.