in-situ conversion to FT disk config?

[Michael ODonnell]

I have a system that boots off a single smallish disk
that isn't Fault Tolerant and I'm content to have it
remain that way because there's nothing irreplaceable
on that disk.  I mention this only to illustrate that I
have a place to stand while I'm fooling with the other
two disks in the system (hda and hdc) which I installed
with the intention of "someday" rigging them in some
FT manner (eg.  RAID1) though I never got around to it.

Well, I recently got that Cosmic Wakeup Call that
everybody eventually gets - a disk on another machine
(fortunately not totally mission critical!)  failed -
so I'm freshly motivated to finally follow through
on my FT plan.  Meanwhile, though, I've accumulated
a fair collection of data on hda while hdc is still
effectively blank, so I wonder if there's some way to
perform an on-the-fly conversion from my vulnerable
single disk config to an FT config that brings that
second disk (hdc) into play without me needing to make
substantial changes to hda.

I'm fantasizing about it being as easy as (A)
installing the requisite package(s), (B) setting up
a config file that describes the desired relationship
between the two disks, and then (C) unmounting hda and
then mounting some virtual device that represents the
newly redundant pair.  I'd be grateful for any pointers,
recommendations, war stories, etc...