I use FireWire with an old G3 and a new MBA as well (and everything in between). Most FW drives I bought as USB2/FW combo units, but I avoid to connect to USB hosts as the data transfer rate is low compared to FW. Getting Sonnet adapter gear to connect FW to Thunderbolt was a big improvement regarding the MBA (no improvement to my purse, however). My favourite drives are the small ones with LaCie rugged enclosure, but they could use some holes for better heat rejection.