I built the Pi this past January, it's been running since then.

Ah, well it could also be an early failure (Bathtub curve), especially if it's a cheap noname SD card.

I'm wondering if the Pi can run off of an external drive enclosure in a mirrored array. I need to check on that.

Several models do support USB mass storage boot. I run several RPis off of SD cards and they've been running fine so far; it really depends on how much you wear out the flash memory.