Quoting from the link below: " . . . any USB device (flash drive, external hard drive, smartphone, digital camera, mouse, keyboard, etc.) that has been plugged into an untrusted computer should be treated with suspicion -- much like a used hypodermic needle. Further, erasing, formatting, or using anti-virus tools will not remove malicious code from the firmware of USB devices. And there is no known method at this time to scan USB devices to see if they are clean."
Read about this serious problem here. Thanks to Jim Hamm for his eagle eye, spotting this vital information.