RL drive select plugs. What is magic about 3?

I've been doing a bit with an 11/23+ setup and a few RL drives, and I've run into something odd. I currently have select plugs 0, 1 and 3. If I plug in two drives, 0 and 1, I can boot from either one, and once in RT11, I can access the other one. Easy. If I connect two drives, 0 and 3, however - I can only use drive 0. All accesses to drive 3 result in an error. Can't boot from it, can't access it from RT11. Thinking that maybe the plugs had to be in order, I connected only one drive, drive 1, and was able to boot it and use it just fine. Then, swapping the 3 plug into that single drive, can't boot from it. And I've shuffled this plug around into a couple of drives, and none of them work with it. There's no damage to the plug itself, and all the drives work otherwise. Unfortunately, I don't have enough cables to connect more than two drives at once. I also don't have a drive 2 plug to try. And yes, I have a terminator. Any ideas? -Ian