So it seems I was a bit quick to declare the USB controller a dud last night.
Overnight, Simon Richter attested on IRC that he also had a USB controller bearing the same PCI ID, and his worked fine. (Although his had the option for extra power from the floppy drive power cable, which I find interesting. Mine does not).
So tonight I worked through the various combinations to get to the bottom of the problem. I put a Sun QFE card in that PCI slot to make sure the riser and the slot were working correctly. It worked fine.
Then I put the USB controller back in and tried a USB thumb drive, that worked.
So I immediately narrowed in on the hard drive itself, and the IDE-USB adapter. I tried it on my laptop. Nothing. Aha. So I tried the hard drive directly with IDE, and it worked. So I assumed that the IDE-USB adapter must have been faulty, and tried a different one. It worked. I cycled through the other three IDE-USB adapters, and they all worked. Then I tried again with the one from last night, and it too worked!
I can only presume that in my haste, or with my fuzzy head (I've got a cold or the 'flu or something), I managed to connect the IDE-USB adapter to the hard drive incorrectly. Don't ask me how, considering it's a keyed interface, but it's not the first time I've managed to screw up plugging in an IDE hard drive.
Anyway, with that hurdle behind me now, I made four physical volumes out of the disks (no partitions at all), and then made a volume group, and a single logical volume that is striped across all four physical volumes (yes, I know if I have a disk failure I can kiss the lot goodbye, but I'm in performance testing mode at the moment). I then exported that logical volume with vblade and mounted it on what will be my MythTV box.
apollock@teevee:~$ df -Th Filesystem Type Size Used Avail Use% Mounted on /dev/mapper/Debian-root ext3 259M 156M 90M 64% / tmpfs tmpfs 502M 0 502M 0% /dev/shm /dev/sda1 ext3 228M 25M 192M 12% /boot /dev/mapper/Debian-home ext3 63G 295M 59G 1% /home /dev/mapper/Debian-tmp ext3 368M 8.1M 340M 3% /tmp /dev/mapper/Debian-usr ext3 4.7G 1.7G 2.8G 38% /usr /dev/mapper/Debian-var ext3 2.9G 897M 1.8G 33% /var tmpfs tmpfs 10M 124K 9.9M 2% /dev /dev/etherd/e0.0 jfs 1.5T 2.2G 1.5T 1% /mnt
I've currently got bonnie++ hammering away at the "disk" to see what sort of performance I'll get, and whether it'll be fast enough to record TV.
Now I just need the TV tuner card to turn up, and I can start fiddling in earnest.