I liked Western Digital's My Cloud network-attached storage device when I reviewed it late last year, but relying on a single-drive NAS can be risky. If that drive fails, and you don't have a backup, you could lose all your data--forever.
WD's My Cloud Mirror solves that problem by putting a second drive in the same enclosure, and configuring the drives as RAID 1. All the same data is written to both drives, so that if one drive fails, you can recover everything from the other.
The model reviewed here came with two 2TB drives, yielding 2TB of storage (not 4TB, because the drives are in RAID 1). There's nothing to stop you from reconfiguring the drives in RAID 0 for blinding speed and 4TB of storage, but that would throw your data-redundancy strategy right out the window. I wouldn't recommend that unless you're absolutely fastidious about backing up your NAS--and nobody is fastidious enough to avoid Murphy's Law.
The My Cloud Mirror will also let you back up its contents to another storage device via its USB 3.0 port, but an even better data security strategy would be to deploy a second My Cloud Mirror (or a My Cloud EX2 or EX4, but it must be a Western Digital device) at a remote location and back up the contents of each drive to the other (you can also do this over your local network, but that's not as safe as having backups at different physical locations).
If you don't want to go either of those routes, WD's software will let you back up your My Cloud Mirror to the cloud (using either your ElephantDrive or Amazon S3 account, though you'll need to pay for whichever service you choose). What it won't let you do is back up a client to the My Cloud Mirror itself over an Internet connection; the client must be attached to the same local network as the My Cloud Mirror.
Unlike a Dropbox account (or Connected Data's Transporter line), which maintain a folder on your local device that is synchronized with your cloud storage, files are stored only on the My Cloud device. The benefit to this approach is that you don't consume the limited storage on your device. The drawback is that you need to have Internet access to be able to retrieve your files.
As with the original My Cloud, Western Digital is marketing the My Cloud Mirror to consumers, and this box has most of the features that audience will want. There's an integrated FTP server, for instance, and peer-to-peer file-sharing (BitTorrent). Finally, there a number of apps you can run right on the box, including Joomla and WordPress, if you want to host your own website.
Western Digital populates the My Cloud Mirror with its own WD Red drives, which are designed for 24/7 operation. The balance of the box's hardware features tilt toward consumer more than SMB. Unlike the more robust My Cloud EX series, this box has just one gigabit Ethernet interface and one power connector, so there's no failover protection on either count.
Consumers will appreciate the My Cloud Mirror's simple graphical user interface, which makes this machine very easy to set up. It comes from the factory with both iTunes and DLNA media servers for streaming media to PCs, smart TVs, mobile devices (smartphones and tablets), and media-streaming boxes in your home. WD provides free basic backup software (WD SmartWare) for your client PCs, and the box supports Apple's Time Machine technology for backing up Macs. Upgrading to SmartWare Pro ($20 per license) adds the ability to back up to non-WD drives and to Dropbox.
WD publishes a number of Android and iOS apps that will help you derive maximum benefit from the My Cloud Mirror. The WD Photos photo viewer is designed to replace your online photo service. You can store all your photos on the My Cloud Mirror and display them on your smartphone or tablet without needing to download the images to your device. You can do the same with your music and videos, although your media-streaming experience will vary depending on your network connection (you'll have the best experience when the NAS box and your device are connected to the same network, versus streaming over the Internet).
As we saw with WD's original My Cloud and its prosumer-oriented My Cloud EX2, the My Cloud Mirror is no barn-burner when it comes to performance. It wasn't terrible at dealing with very large files (we test read and write performance with a single 10GB file), but it was considerably slower reading and writing our 10GB collection of files. And that will likely be the more common real-world usage scenario.
If you think you'd benefit from the additional features that the prosumer-oriented My Cloud EX2 has to offer (dual Ethernet, dual power-supply inputs, and 10 licenses for WD's SmartWare Pro), that box is street-priced just $18 higher than the My Cloud Mirror. The SmartWare Pro licenses are probably worth it if you have enough Dropbox capacity to take advantage of that feature, but few consumers will. I imagine even fewer will be able to take advantage of the EX2's additional hardware features.
The original My Cloud is a very good--if a bit slow--consumer-oriented NAS box, and the My Cloud Mirror adds a valuable feature in RAID 1. If you use your NAS to store critical files, this is a solid buy.