I just managed to get a VIA 8237 fakeraid mirror set up. It all works beautifully. The only pain is that the original disk devices and partitions in /dev are still visible. I need to work that out...
for the interested, some steps:
1. I first installed Linux (Mepis) onto one of the SATA disks, with the other disk disconnected.
2. I then shutdown the PC and connected the mirror disk. When booting, the controller discovered the extra disk, and gave me the option to rebuild the mirror set. This takes some while!
3. After lots of looking around, I stumbled across this page: https://wiki.ubuntu.com/FakeRaidHowto
It describes how to setup a striped (raid 0) set, but it contains a link to a page that gives the setup for raid 1
4. I edited /boot/grub/device.map and /boot/grub/menu.lst. They now look like this:
(hd0) /dev/hda (my IDE disk)
(hd1) /dev/mapper/via_ddcafcgade (the DMraid created device in /dev/mapper)
title MEPIS mirror disk
kernel /boot/vmlinuz-2.6.15-1-586tsc root=/dev/mapper/via_ddcafcgade3 nomce quiet
initrd /boot/initrd.raid (this is the initrd image I created, which loads dmraid)
Basically, that's it!
Now, if someone knows a way to get those old devices out of the list, that'd be great...