WD says it uses RAID-optimized WD RED drives, which spin at 5,400 RPM, though it doesn't specify how many. The box includes a USB 3.1 Type-C interface port and two USB 3.0 Type-A ports that can be used to connect flash drives and other accessories to your PC. They can also charge up your smartphone, letting the drive do double-duty as a USB hub (it comes with a USB-C to USB-C and USB-A cable in the box). The My Book Duo also supports 256-bit AES hardware encryption.
The base 4TB model costs $280/£270, while the top end 20TB configuration will set you back $800/£620. As a point of comparison, Samsung just unveiled its portable T5 SSD that costs $800/£760, but you get one-tenth the storage -- 2TB. However, you also get a lot more performance, with 540 MB/s from a single drive and the greater reliability of SSDs. In other words, price is the lone advantage left to mechanical disks and the reason why WD is so anxious to keep its relationship with flash storage manufacturer Toshiba.