I'd like to use a shared drive on my Airport as a central location for all of my media. But, I'd like to not place all my important data on a single drive if I can avoid it. Is there any way to build in some kind of redundancy or RAID as a kind of wannabe SAN into something Airport will allow me to share as a single disk, on the cheap/free?
According to this Apple support article:
So it would appear that you need something that could be connected to the Airport Extreme that does its own RAID. Perhaps something like the Netgear ReadyNAS Duo, if you want to get a ready-made solution. You can also buy NAS enclosures with two or more bays, a couple of bare drives, and roll your own. Note that you would typically connect your NAS to the Airport via Ethernet (though some NAS enclosures allow you to connect via USB, which was your original plan), but since the limiting speed is the Airport wireless speed, I don't think that would be a problem.