The problem here is if you are not paying attention (read the AC manual AND your MB manual) then it is really easy to hook them up incorrectly. On almost every MB I have ever seen the USB Pin 1 is always hot. However all my Aquaero USB connectors have Pin 5 as hot.
Always make sure you know what you are doing when hooking up electrical connections or woe be onto you as Shoggy has pointed out.
BTW, it doesn't matter whether you have the hot and negative leads reversed on a 3 wire flow connector providing the sensor wire is hooked up correctly on AC stuff (don't know whether this would be true on other flow meters).