90% of the market using a desktop computer has requirements over and above anything these iOS devices can do. How much RAM is in the latest iPad now? 1GB How much is in the typical newer Mac or Windows PC today? At least 4x that much! How about video capabilities? You *really* think the iPad's video processor is anywhere near as capable as a mid-grade (never mind a "high end") video card from nVidia or ATI?
IMO, this has never *really* been about tablets supplanting desktops. It's about a world where you have more OPTIONS. If you really don't do much more than consume information (such as reading e-books on the go, checking email and surfing the web), an iPad is probably just what you need. If you actually write your own software (or heck, even design web pages), it's FAR from adequate.
To use a construction analogy, the iPads are a little bit like having an electric jig-saw. Very useful tool in its own right, and depending on the work you do, might be all you really need. But quite likely, you also need to cut a lot of 2x4s and other such "structural wood". A jig-saw's blade isn't long enough for some of that, and it would take too long, and be harder to achieve a straight cut, in other cases. Think of a desktop PC as your circular saw.
All in all, I'm not sure I'd even care much about what you're proposing here; a way to interface a large hard drive to my iPad? I can imagine it'd be useful in a few niche cases, such as photographers who need to change up what's actually displayed on their iPad, on the fly, from a large "master collection" of images on the external drive.