I have it completely unplugged from mains and USB, but when I slide the gantry (in either x or y) the steppers must be generating some back current that powers the laser on. Is this normal?
The faster I slide it, the brighter it gets, and it gets pretty bright!! Like, I should be wearing goggles bright. Major safety issue. Not expecting that when it’s unpowered.
Is this just me or is this a known issue? With a known fix?
Sculpfun S9.
The fix is: “don’t move the gantry by hand” - you are generating electricity by forcing the motors to turn, and that electricity has to go somewhere or something is gonna explode.
That back emf can cook your motor drivers and or the control board. By moving the gantry fast enough to produce a “powerful” light - you are going to cook something. The expensive parts usually cook before the cheap ones.
OK, good to know it’s not just me then.
Nope, not just you. My 20w - 40w LED’s at the switch light up on my laser module.
The expensive parts BURN UP TO PROTECT the cheap ones. 
This is what is called a reverse generator.
As @JimNM describes, don’t do it, it (can) ruin your machine.
Hi @pag
This is a fun one!
That makes sense — the steppers are effectively acting as generators when the gantry is pushed by hand.
One extra point is that “unplugged” does not always mean the motor circuit is electrically isolated from the rest of the machine. The generated voltage can still find paths back through the stepper drivers, protection components, shared power rails, or the laser module circuitry, which is why LEDs or even the laser can briefly light.
Better isolation/protection is possible, but a lot of these diode machines are built to a price, and the design assumption is usually that the motors are driven by the controller rather than spun by hand. So the safest practical advice is: don’t move the gantry quickly by hand when the machine is off. If you need to reposition it manually, move it slowly and gently, or use the jog controls with the machine powered.
That said, it’s your machine, and who know, with energy bill costs, hand-cranking your burns might save money! 