Here's how it works. The PhotoFast GM2 SFV1 Air Upgrade Kit for the MacBook Air comes with a 256GB module and a USB 3.0 housing. Once you've swapped the 64GB internal module with the new one, just pop it into the USB housing and presto! You've got 64GB of external storage.
There's no word on pricing or availability yet, but we're guessing it won't be cheap. Still, it's a nifty little device.