Wi-fi turns off when USB device is plugged in

As of this post, two people have posted some great possible solutions.

If those don't work, or you want to try something else first, try attaching only one very low power USB device into the port (such as a USB flash drive, but not a mechanical drive). See if your Wifi still works. If it does, that would indicate that it is a power issue, and that the power drain from your USB port is causing your Wifi adapter to fail. It's an easy-to-check possibility.