I was transferring data from my wife's old computer to her new one. Because I am lazy, I had the mouse and keyboard plugged into the front USB ports of the old one so I could switch them back and forth from box to box easily. So when I plugged the external hard drive into the old one to extract the data, I had to do it in the back. I could not figure out why the machine could not find the drive when I plugged it in.

Turns out, an RJ45 jack is exactly the same width as a USB jack.