To Help Troubleshoot My Horizontal Lines on Monitor Issue

Hi, I have quiet an issue i would help with, whenever i plug in a usb hard disk, my monitor starts showing small horizontal lines all over the screen but mostly on the upper part. It is not a video card issue because i tried this on several computers, several video cards, and 2 different monitors. Basically its the same results, whenever i switch on a usb external hard disk the screen get the lines. The issue seens to be that whenever i have the monitor wire hooked with the computer and the usb at the same time - the prolem happen. Would like any insignt to sort out this issue Thanks Jo