Your correct about being able to use a Laptop with an external Motion controller BUT you still need to set it up so the Energy saving features are turned off.

This could be your problem.? Laptops are some times setup to turn off USB ports off when not used. To be Honest I hate USB, more so than Parallel port because it's always dropping out or fluctuating power which causes problems like this.

Personally after checking all wire connections for loose or broken wires I would turn off all power saving and Virus checkers etc on laptop. Then for good measure make a Backup of your XML file then re-install Mach3 and the Motion controller Plug-in.