hot plugging

Hot plugging, or hot swapping, refers to the ability to add or remove hardware or peripheral devices to a computer while the computer is running. The operating system automatically recognizes the change and the system isn't interrupted. 

Computer components are usually described as "cold-pluggable" if the computer system must be powered down to add or remove them. So if something is "hot swappable" it means you can remove a drive without powering it down. 

A well-known example of this functionality is the Universal Serial Bus (USB) and FireWire that allows users to add or remove peripheral components such as a mouse, keyboard, or printer. It's also a feature of PCMCIA.