After a good bit of trying to figure out what was wrong, I discovered that the +5 volt pin in the power connector I was using for the #2 (B) drive had pushed out of the nylon plug so that the drive was unpowered. Having either of the drives unpowered on the cable caused the other drive to screw up. The two drives cooperate just fine when both have power. I don't remember ever seeing this kind of problem before... I seem to remember having unpowered drives hanging off of cables with no ill effects. I guess that some of the signals (write gate for instance) must be getting pulled low by the unpowered unit. Is this normal floppy behavior and I'm just remembering wrong?