I have been working on a scratchbuilt printer using a RAMPS 1.4 board and using a 12v ATX power supply scavenged from an old PC. When I started tuning it, it worked fine, but one day, when I plugged in the power supply, the current traveled up the USB cable and destroyed my hard drive, as well as my Arduino. This was not the first time that the printer was plugged into the PS as well as my PC. How can I prevent this in the future?