The two-bay Western Digital My Book Live Duo 4TB covers the basics well, and does so for a lot less than most competing NAS boxes. Priced at just $380 (as of March 23, 2012), it's easy to configure, provides all of the sharing features that most home and small-office users need, streams media well, and has strong backup capabilities. It also provides access to files from mobile devices, and it turned in very good performance when reading large files.
The My Book Live Duo ships in maximum capacity mode, meaning that the twin 2TB drives are configured for spanning, or JBOD (Just a Bunch of Disks), where the network uses the second drive only when the first it full. This is a bit safer than RAID 0 striping of data across both drives, where one drive's failure means the loss of data on both. But it's not as safe as RAID 1 mirroring mode, which the My Book Live Duo also supports. Mirroring reduces capacity by half--to 2TB in this case--but it also enables you to continue using the box if a drive fails.
Western Digital provides mobile access to shared files on the My Book Live Duo via its WD2go Web portal. The company offers free apps for both IOS and Android devices, though the pro versions of those app, which store files locally for offline use, cost $3 each. You may also directly access the My Book Live Duo's HTML interface remotely, as with most other NAS boxes.
Of course, getting 4TB of storage for the same price as many driveless, pro-level boxes means that you won't find many advanced features. The unit supports USB 2.0 but not USB 3.0 or eSATA, which makes the process of backing up the box more tedious. It also lacks Web serving, one-button copying of flash drives, and Rsync for mirroring data with remote boxes. On the plus side--and new for a consumer-grade WD box--the My Book Live Duo makes it easy to swap drives by popping the top cover and pulling out the drive tray.
With an 800MHz PowerPC CPU and 256MB of memory, the My Book Live Duo turned in an excellent large-file read performance, handling our 10GB file at 91.1 megabytes per second. In other tests, it fell more in line with similarly priced boxes from Buffalo and LaCie, writing the 10GB file to its platters at 46 MBps, reading our 10GB mix of files and folders at 36.2 MBps, and writing that mix at 28.1 MBps. The large-file read speed bodes well for streaming audio or video to multiple locations, but it makes WD's NAS box less suitable for backing up or copying large amounts of data.
The My Book Live Duo is great for home users who want to stream media; but small-business users will be better served by a box that delivers faster overall performance and has more-advanced features. Note: Seriously consider switching the box to maximum protection mode (mirroring) if you plan to store anything irreplaceable on it; what you sacrifice in capacity you'll gain in peace of mind.