I have an iPad. I also have a Google Nexus 7 tablet (and yes, it basically fixed everything that was wrong with my Samsung-delivered sub-par Android experience). Both tablets are really optimized for slightly different tasks. However, they share a common Achilles' Heel: neither of them is designed to accommodate additional storage. If you bought one of these tablets with 8Gb or 16Gb you're right out of luck if you want to carry 32Gb of stuff around with you.
So I went looking for a solution, and found one.
What I wanted (and went looking for): a box, about the size of a pack of playing cards or cigarettes. It should include a battery, some sort of storage medium, a USB port for shovelling files into it, and enough smarts to attach via wifi to a network and make the files available via a relatively open API such as HTTP with WebDAV for saving updates. In other words, a small network-attached server for files I can carry in my pocket and that overflow my tablet's own storage.
Why not use DropBox or Box.net or iCloud?
Well, I'm not always attached to the internet. In particular, if I travel abroad, then unless I'm in range of a wifi hotspot, the cost of international roaming data is insane. (On recent visits to the USA I've been charged up to $6/Mb.) Again, even if I've bought a local PAYG SIM for my mifi or iPad, reception may be lousy—at one recent trade show, the T-Mobile network was so swamped that even 2G data service was effectively unavailable. Finally, cloud services can fail, and when they do, it's more than a little inconvenient. So a box that stores a bunch of stuff in my pocket seems like a useful adjunct to life with a tablet that can't take micro-SDHC cards.
My preliminary research didn't look promising: the Seagate GoFlex Satellite looked close to what I wanted, but failed on numerous levels. It's a battery-powered portable USB hard drive that broadcasts a wifi hotspot and permits streaming of content to attached devices: close, yes? But closer examination revealed a whole lot of fail. It appears to require a wall-wart to charge its battery, even though you use USB to move files onto it. It only talks to a proprietary app (versions for iOS and Android), apparently only serves up media files that the app understands, is formatted with NTFS (not so friendly, Apple/Linux world), and you can't save files to it from the iPad. Oh, and it can't attach to an existing wireless network. So you can use it to play movies or music, but that's about it.
I first looked into the Satellite six months ago: back then it appeared to be the only player on the field. I confess I was annoyed enough I even discussed the idea of a DIY project, probably kickstarter-funded, with a local entrepreneur: Raspberry Pi hardware, SDXC storage, a battery, and a custom case plus Linux-based OS. But luckily other people got there first, thus saving me a lot of time and money.
To cut to the chase: over the past couple of months a whole bunch of tiny battery-powered NAS devices have come on the market, and I looked into a bunch of them before buying one. The Kingston Wi-Drive looked promising, as did the Transcent StoreJet, but both suffer from the twin drawbacks of creating their own wireless hotspot (you're cut off from the internet while using them) and of being non-expandable FLASH devices. The HyperDrive CloudFTP looked very like the device I'd considered building; it's a kickstarter-funded startup project, but one that succeeded. You supply the USB storage medium, and the CloudFTP box makes it available as attached storage to whichever wifi network you configure it to connect to, or directly from its own hotspot. It's physically chunky, though, and reports on the software quality are equivocal.
But In the end, I went for the Maxell AirStash, for reasons I'll discuss below.
The AirStash looks at first sight like a chunky SD-card reader (about the size of a disposable cigarette lighter), with a USB 2.0 plug at the opposite end from the SD card slot. But there's a magic button. Push the button, and it creates a wifi hotspot and serves up the contents of the SD, SDHC or SDXC card (up to 2Tb). Purchasers of big SDXC cards should note that it wants them to be formatted as Fat32, not ExFAT—it took me a tooth-grinding hour to figure that out (the hard way). Files are accessible via a proprietary iOS app (no Android version available), or via WebDAV, over the network created by the AirStash.
However, there's some new firmware in beta that makes it much more interesting. Upgrading the firmware is easy: drop the upgrade file onto an SD card, plug the AirStash into a power source (but not mounted as mass storage by a computer), and it auto-installs. I'm currently running the June 7 beta 2 release, and it adds what Maxell call "SideLink"; you can configure the AirStash via the iOS app to connect to an existing wifi network. So, for example, if you carry a mifi you can configure the AirStash to connect to the mifi's network and serve up files to other attached devices without requiring them to connect to its own network (which by definition isn't internet-connected).
Note that SideLink is not configurable via the web interface, or via any mechanism other than the iOS app, at this point—but the basic functionality is still in beta; I expect web configuration will come (if only because it's a no-brainer selling point to Android tablet owners).
Also note that this feature is in beta. When it works, it's great, but today it's been showing a worrying tendency to forget the passwords to my preferred networks. If you really need this to work right now, out of the box (for example, because you need it for a non-technically clueful friend), you'd do better to wait a couple of months.
My take is that SideLink and the ability to expand its storage via SD cards are the killer features that make the AirStash far and away superior to the other pocket wireless NAS devices I've seen so far. While the beta software is still, well, in beta and a bit wobbly, it shows promise. Oh, and it works fine as a (slightly big) SD card reader for any USB-equipped computer. Most importantly, it fixes a design flaw in the best-selling 7" and 9.7" tablets, which between them account for around 80% of the tablet computing market. I'll certainly be carrying it in my gear bag until something better comes along; and if the SideLink feature stabilizes in the next couple of months, it may well let me replace my current more-than-two-year-old iPhone with a new one with less than the maximum amount of storage (at which point it will have saved me more money than it cost).