Well, we've just been dumb here. There are IDE drive cables that have pin 20 covered to stop them being put in upside-down. Well, we've gone and done just that (don't ask how, I can't even figure that one out right now), and while the drive appears to still be working I'd really appreciate it if someone knows where to find a list of what pin does what on a standard 40-pin IDE interface. I dread to think what we've just broken. doh!