I forget if the macbook air has an easily removed harddrive -- I know with the older pros it was super easy. Here's an easy solution that's not very involved at all:
1) You could get a large + fast SD card and just plug it into the side:
Leave-In Compact SD Cards Boost Your MacBook's Storage Capacity
2) Or, you could buy this adaptor for your SD card slot (You have to have the bigger macbook air for it to work
check here)
Amazon.com: The MiniDrive | microSD Adaptor | MacBook Air/Pro/Retina: Computers & Accessories
and pick up any microSD card from 64-128 Gb for cheaper than the PNY card + it sits flush with the side so it will look OEM.
You can shop around but here is an example of a memory card that would work with the adapter I posted:
http://www.memoryexpress.com/Products/MX52300
For $40 + $20 Adapter you can have 64 gigs of fast removable storage which will sit flush inside your laptop and requires no opening of the laptop. That's kind of a crappy memory card i linked to, but if you shop around you should be able to find better quality with the same size/speed for a comparable price
As long as the quality of the card you use is pretty fast (class 10 or something similar) you should have pretty fast transfer rates. If you want a laptop/portable computer I'd stick with what you have -- it's one of the best lightweight laptops IMO. I've had my macbook pro 15' since 2011 and it's a heavy mofo, I'll likely be upgrading to an air/retina in the future as well