No one seems to have built one of these yet. Smartphones and tablets are getting more popular and many of the newer ones now support USB OTG in host mode allowing you to connect a usb drive with an appropriate adaptor. But it’s cumbersome, ok for if you’re wanting to copy some files off your camera or something, but not handy for keeping videos or other media on, as a supplement to the onboard memory / sdcard (it would be particularly handy in that many phones require you to remove the battery to swap SD card, and now that android installs apps to the SD it’s not really practical to swap them around)
What I’d like to see is a usb drive, not much bigger than the cruzer blade, with a standard USB A male connector at one end, and a microUSB male connector at the other (with the appropriate bridge for OTG compatibility) ideally in some sort of flip arrangement so that the connector can either be hidden away or deployed at 90/180/270 degrees to allow the best chance of connecting it and getting it flush against the side of various devices.
I think this would be a really handy device, and while the market for it might not be all that big right now, it will continue to grow. You’d be able to sell it at a premium, especially being the first company to bring it to market, and the additional cost in manufacture probably wouldn’t exceed more than $2.
You’d obviously have to build some sort of protection against people plugging in both ends at once, but for added value, if you could sense this you might be able to switch it to simply pass-thru making an ultra compact adaptor/cable which could be used to connect your phone to your laptop or to an emergency charger etc.