What is activation key of USB disk security?

USB activation keys are a common solution used to increase the security around the use of USB storage devices such as as memory sticks and external drives. The keys function to prevent the spread of malware from such peripheral devices to the machines that they are plugged into. Thus a USB activation key will be used by software to prevent the spread of malware to a USB stick that is not as yet infected and similarily prevent the spread of malware from an infected USB device to something like a computer.