Just had the displeasure of figuring out the positive battery terminal has completely corroded away from the battery. When I opened the cover for the positive terminal, the entire piece fell off of the battery. So my question is, how do I safely remove the negative terminal to replace the positive terminal? I've spent a good deal of time trying to "Google" it, but can't find anything other than to remove the negative side first.
On negative ground vehicles (virtually all cars now), the reason you should remove the negative side first is so that if your wrench is touching the neg terminal, and it makes contact with the body/engine/frame/etc, then it is safe because the neg terminal is all ready connected to those. However, if you try to remove the positive first and you wrench makes contact with the body, you create a dead short and make lots of sparks, start a fire, etc.
Now in your case, the positive is already disconnected, so it is safe to remove the negative.