I have a board design inside a metal case (video processing MCU), and it has one external USB port with ESD protection on all pins. It's battery powered and handheld. The exterior metal case is not tied to the ground of the internal circuit board. In fact the board essentially floats inside the metal housing on plastic standoffs, with the processor pushed up against the metal through a gap pad for a heat sink. I don't have the option to go to a plastic case.
The problem is that occasionally when someone touches the unit, or hands it to another user it goes bonkers. I suspect ESD, but I'm not sure of the right path to fix it. If it were a plugged in device I'd do ESD protection to ground. But in a portable case I don't see where I can redirect the energy to. I'm left thinking about using an ESD protection coating that greatly increases the resistance of the outer case.
But is there another option I'm not thinking of?