Holding up a tablet at a wedding or party to take snapshots: fair game, or just lame? The etiquette jury's still out on that score, but wherever you fall on the issue, we bet you've used your tablet or smartphone to create photos, videos, data, or files you just can't replace. And, just like any other bundle of bits you may have created on a desktop or laptop, our first question is the age-old one: What are you doing to back it all up?

If you're like too many of us, the answer is "nothing."

Now, if you have your tablet or smartphone configured to auto-upload your created media to the cloud or share it with your social-media accounts, maybe you don't need to worry. But if you take lots of video or photos with your mobile device, and you would be crushed to lose it all were the device stolen, broken, or dipped in the sea, you need a backup plan. Enter one possibility: a wireless drive.

Wireless Drives: The Landscape

Note that portable, wireless hard drives are not just for backup. Yes, they let you back up precious content from devices that don't typically take a direct data-cable connection, but they can also act as storage extenders and streaming-media sources for your tablet or phone.

Most tablets and phones come with just 32GB, 64GB, or 128GB of onboard storage, with, if you're lucky, a little wiggle room for expansion via MicroSD card. (Forget about that if you own an Apple i-anything; they're all slot-less.) One of the wireless drives profiled here can come in handy if you want to, say, carry a library of movies on a trip to watch on a phone or tablet, but don't have the room in its local storage. (You access the content, or transfer it to and fro, using vendor-supplied apps.) Plus, they'll also work with ordinary PCs equipped with a wireless connection, so you can pair the drive with your laptop over wireless--or you can just use a USB wired connection, like you would with an ordinary external drive.

Some wireless drives employ flash memory, but most use standard 2.5-inch platter hard drives inside, the same kind of compact drive mechanism used in laptops. As you might guess, platter-drive-based models offer far more gigabytes for your dollar, but the flash-based ones are better able to survive jostling and bumps while operating. We've looked at a bunch of models on the market, of different paradigms, even including one solid-state (SSD) model from WD, using fast NAND memory inside.

Most of the drives profiled below are, at their heart, mobile hard drives in an external chassis, equipped with a battery for on-the-go operation. Most also let you pass your Wi-Fi connection through the device, so it's possible to maintain wireless Internet access while remaining connected to the drive.

A related wireless-drive paradigm is the excellent and flexible Kingston MobileLite Wireless , which acts like a wireless drive without the hard drive, letting you turn any USB key, USB hard drive, or SD card into a wireless storage drive. If you own a host of existing USB storage, this could be a cheap and easy way to cobble together a wireless-drive equivalent without actually buying one. You can use it for occasional wireless-drive backups without investing in a unit with its own storage. And another still-different one is the Lenovo ThinkPad Stack (listed above), which is a mobile-accessory set that includes hard drive, power bank, and router modules that together can work like a wireless hard drive. It's been on the market a while, but it's a unique solution for serious mobile types.

Buying a Wireless Drive: The Basics

A few key things to look out for in any wireless drive...

SD, or No SD? A few wireless hard drives include an SD card slot built into the body of the drive. Having this slot can make accessing (and/or backing up) the contents of a camera card easy.

Weigh the Raw Capacity. Any of these drives that uses an internal platter mechanism will cost less per gigabyte than one based on flash memory. You'll want to do the math on the cost per gigabyte on any drive (total capacity in gigabytes, divided by the cost in dollars) that you are considering to assess the relative value.

Consider Battery Life. This is one way in which our formal reviews of these devices are invaluable. Check out our battery tests on the various drives to get an idea of relative longevity if you do mean to use these for extensive streaming of music, photos, or video away from a power plug. Also, realize: It's not an absolute measure by any means, but the bigger-chassis drives tend to have larger batteries and thus longer potential running times. A few can also serve as power banks, recharging your mobile gadgets off the drive battery.

Look at the Software Interface. This is another front on which reviews will come in handy. The app aspect of the wireless drive will vary depending on platform (iOS versus Android), and some experiences are better than others. You'll want to look at the particulars. This is mainly an issue for how the wireless drive connects with phones and tablets; we expect that most users, when connecting one of these drives to a laptop or desktop, will use the wired connection and manage the contents of the drive using Windows Explorer or the macOS file manager.

Check the Streaming-File Support Specifics. If your main reason for considering one of these drives is for it to act as a streaming source for music or movies, look into which types of files you can play through the provided app. If your collection consists of files in less-common formats such as FLAC, you'll want to be double-mindful of this.

Know the Maximum Connectable Devices. Wireless drives vary in terms of how many tablets or other mobile devices you can connect to, or stream from, them at any given time. If, say, you intend to stream different content to a host of kid-controlled tablets in the family minivan, pay attention to this. Also know that the kind of content you are streaming can affect the smoothness of playback. Three simultaneous MP3-music streams are a lot different, in terms of demands on the drive, than three streams of 1080p video.

Ready for Our Recommendations?

You won't find a ton of these kinds of drives on the market, but we've tested most of them. For info on more ordinary wired portable drives, check out our roundup of the best external hard drives, as well as our guide to the best external SSDs.

And for additional backup options, also take a look at our buying guides and favorites lists for the best network-attached storage (NAS) devices and cloud storage services.