While I don't use an Air (MBP here) I guess it depends on your needs. You can get 128GB flash drives for 50-75 bucks now which should handle just about any backup needs while on the road. Of course for a little more you can get a much bigger HD but it takes up more room in your bag, adds weight, can be easily damaged if it gets dropped or banged around, etc. On the other side of the coin cheap flash drives tend to be slow to write to and are easily to misplace or lose compared to a standard drive. If it were me I'd probably go with a 64 or 128 GB flash drive to use as a backup while on the road until I can make a permanent one at home and have another that's bootable with all the apps and stuff I normally use just in case something happens to my main system.