The Western Digital My Book Live Duo is network attached storage that lets you combine two hard drives for 4TB or 6TB of storage, or set up a mirrored RAID 1 configuration to make a second copy of all your data. While that protects your files in case one drive fails, it also reduces your storage capacity by half. The Duo does all the other modern NAS tricks too, like DLNA streaming, automatic backups, and mobile access through the WD 2go app (Android 2.1 or higher and iOS 3.0 and up). You also get a USB 2.0 port to extend capacity or transfer files. Pricing starts at $399.99 for 4TB and $499.99 for the 6TB version — that's about mid-range, as 4TB NAS boxes run from around $300 to $700.