Need help using 2.5" HD on Rev.3 Beige G3 mobo

I'm trying to use an IBM 4GB 2.5" drive with the G3 desktop mobo and parts I picked up a while back at an auction. The aim is to stuff everything into a hollowed-out Mac Portable shell and run it off of a standard brick or a car's 12v power (via a specialized Mini-ITX power supply).

But!

9 times out of 10 (or more!) the system doesn't even see the drive. I'm using the original IDE cables, which work, and the original HD, which works, and I've switched both drives to every position, plus added or removed both the CD-Rom and floppy drives. Here's what I *do* know:

1) It's not the drive, that works fine in my Shuttle box.
2) It's not the cables, they work fine on any other drives.
3) It's not the 44-pin-to-40-pin adapter, both of those work on my shuttle box.
4) It's not the OS, exactly: I've gone from OS9 up to 9.2.1
5) It's not Drive Setup, 'cause I hacked that per a nifty web page to allow unknown drives from known manufacturers to use the base driver.

I'd prefer not to have to buy a fancy IDE card just to get a 4GB notebook drive to work, since that A) costs money, and will require getting a dual-, rather than single-, PCI riser. I'm going to have enough trouble as it is getting the PDS card kludged up to lay sideways.

The hell of it is, I *had* the drive working. Per Finagle's specifications, nobody was around so I have no proof; I was using a USB drive to run an OS 9 patch, and when I booted the beast up I could *hear* the drive start chittering. Normally it just sits there, spinning but not accessing, but this time it was detected. I used the hacked version of Drive Setup, initialized the drive, copied the whole of the old drive contents over, rebooted after unplugging the old drive, and it worked!

Then I had the bright idea of hooking the CD-Rom back up, and the new drive hasn't worked once in the last 16 hours.

Help!