AirStash ($99.99) uses Wi-Fi to form a bridge with a destination device; it is the only of the four devices discussed herein that uses any local storage, but even that depends on the SD card used with it.
While it's not unusual to find smartphones and tablets with 16 or even 32 gigabytes of storage today, there's sometimes a need for more plentiful or more flexible storage.
The black and green AirStash looks somewhat like a double-wide USB memory card reader/writer, and can work as one. However, once you unplug it and press the power button, it acts as a miniature server to any Wi-Fi device that has a browser. You can access it via an iPad's Web browser and look at photos and even play videos using the iPad's larger screen.
Also, there's no need to involve the PC; AirStash can read SD cards directly from most digital cameras.
While AirStash will work with most modern smartphones, its user interface is designed to look like an iPhone app, and you'll get a few hours of use out of its necessarily small battery.
Also, just as with the Wireless Media Stick, you must make sure the client device can read the file format, and you may not have much luck streaming high-definition movies, but standard-definition moves should work fine.
Contrary to what its telltale vowel-starting name might imply, the iTwin ($99) won't duplicate an iPhone or iPad.
It doesn't even work with them (or even iMacs although Mac support is in the works). A two-headed USB stick available in gray or metallic green, the iTwin operates somewhat like the Wireless Media Stick; you copy a folder or even your whole hard disk, to the drive so it can be accessed from a remote device. However, there are a few differences.
On the plus side, unlike the Wireless Media Stick, which works only within a home network, the iTwin's live link back to the original PC can work across the Internet, so you can access your files from almost any place you have broadband access.
The company aptly describes the product as two ends of a cable without the cable.
However, there are some limitations. First, one half of the stick has to stay connected to the source PC, and iTwin can work with only one source PC at a time.
As with the Wireless Media Stick, that PC must stay on and connected to the home network. Unlike as with the Wireless Media Stick, though, iTwin can be used only with PCs as the destination.
And as with other products that provide a path back to you home PC through the Internet, access may be even slower than your broadband connection usually is. This is because iTwin depends on the data speed from your home up to the Internet, which is almost always slower than the speed down from the Internet.
Despite these limitations, the iTwin system is easy to understand and use, and worked well with firewalls that sometimes caused problems for other remote access products.