There are several solutions to the problem you have described. First, when the watchers appear, you can pretend that the machine got "broken" so they have to wa

There are several solutions to the problem you have described. First, when the watchers appear, you can pretend that the machine got "broken" so they have to wait until it gets "fixed" (reflashed with original firmware). Of course, you can "fix" it once again after they leave. Second, you can make it return original firmware when cloning the disk for verification or store modifications in RAM only. Third option, you can have two disks, one with original firmware for verification and other that is actually used. Fourth option, you can install additional microcontroller and connect it to the printer so that it prints what you want instead of actual results.