What's the error message?
Can the BIOS autodetect it?
Does the harddisk spin up? (if not, you got the cable plugged in in the wrong direction)

There is no error message. The BIOS "POST screen" says C drive: no disk. The BIOS is unable to automatically detect the HDD - it finds nothing. If I enter the HDD's details manually (ie, cylinders, heads, sectors), then the mobo hangs as it attempts to find the HDD. Finally, the HDD does spin up OK. The ribbon cable seems to plugged in the correct way.

Over 15 years ago I had an AMD 486DX4-120 system with a VLB motherboard (PCI was new and unproven, so we made the mistake of going with the short-lived VLB when we were looking at motherboards; oh well, it was a 486 in an era where everyone smart was buying Pentiums anyways hehe). We got a VLB IDE controller for it and eventually a VLB video card. Unfortunately it was so long ago that I can't remember any details about the controller.

If I had to guess, though, there's probably a BIOS for the VLB IDE controller that gets called by the system BIOS (or vice versa?) and maybe you can get to a setup utility for the controller's BIOS to configure the connected hard drives?

Hmm, maybe not. First Google result for QD6580W shows a lot of jumpers (old school!): http://stason.org/TULARC/pc/hard-disk-floppy- … 80W-757-77.html

Have you tried messing with those jumpers on the VLB IDE controller card?

I think the jumpers are OK. I've even got the printed leaflet for it! But I still can't get the HDD to boot. I removed the VLB controller card, and tried two different PCI "100 speed" IDE cards. Neither worked. So I dug out an old ISA multi IO card, and that boots up a very old HDD OK. Oh well. 😐

Solved! I got the VLB IO card to work. One of the ISA contacts on the card had some crud on it. I scrapped it off with my finger. I reinserted the card, it works, HDD boots up OK. 😀