Lugging around an external drive that weighs more than your MBA kind of defeats the purpose of buying a lightweight laptop. Luckily, Western Digital's latest My Passport drives can hold a terabyte of data but tip the scales a less than a pound.
The Studio connects with FireWire800 for lightning-fast (though not Thunderbolt-fast) data transfers and the ability to daisy chain other FireWire peripherals or hard drives. It also features an e-ink display that reminds you what's on the disc without having to plug it in or take a Sharpie to the case as well as 256-bit hardware encryption. The My Passport for Mac also has a 1 TB max capacity but does away with the display and FireWire in favour of the smallest footprint ever for an WD external drive -- less than 3/4-inch thick -- and is powered through the USB 2.0 port.
Both drives are formatted for Mac and are compatible with Time Machine. If you're not a fan of TM, you can also use Western Digital's Smartware system which, after its initial setup, will automatically update the backup image in real time whenever a file is added or modified. The My Passport Studio runs between $US130 for the 500GB up to $US180 for the 1TB. The My Passport for Mac costs between $US100 and $US130 for the same data capacities. Both are available on the Western Digital website.