Your M3 3D Printer has a built in computer, the Raspberry Pi. Just like a normal computer, you need to shut down your 3D printer before removing power. You will also need to shutdown your computer if you want to use the USB port on the printer controller.
There are 2 ways to shut down your printer.
1- Shutdown Via Octoprint
Octoprint will allow you to shutdown the printer from the Power Menu. Click once on the power icon at the top of the interface to open the Power Menu. Now click 'Shut system'. The Octoprint interface will tell you that the system is unavailable.
2- Shutdown via Reset Button
On the front of the printer there is a pushbutton. Use an Allen key or other small object to push the button and hold it down for 10 seconds. When you let go, you should hear 2 short tones followed by 4 more tones. This means the system has begun to shutdown.
Using both methods it will take a minute for the system to shutdown. The the green light on the printer has turned off, the computer is shutdown and your system is ready to be powered down.
If you need to restart your printer after shutting the computer down, simply power-cycle the printer by flipping the power suppliers switch off and on.