That's my current solution. 512GB SSD in my MBA and a 2TB Seagate Slimdrive. I created two iTunes libraries and am using the persistent ID hack to make my iPhone/iPad think they're the same one. I have all my music on one (on the SSD) and all my other media (movies, tv shows, apps, podcasts, etc) on my external drive. When I start iTunes I hold down the option key and choose which library I want to use.
Not pretty.
I also tried not having iTunes not manage my files but that creates a whole lot more manual work when bringing in new files. Also, the iOS apps have grown to over 40GB. It's insane.
Then there's the issue of having to tote that drive around when I'm on the go. Kinda defeats the purpose of a thin laptop.
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Yes, this is the same conclusion I am reaching. A 15" fat MBP or a 13" MBA and external USB drive.
Sigh.