What technology allows devices to be added without turning off the server?

The concept of adding devices to a server without shutting it down is best described by hot plug technology. Hot plug functionality enables the insertion and removal of hardware components while the system is still operational, ensuring minimal disruption to services. This capability is crucial in environments that require high availability, as it allows for hardware upgrades or replacements to be performed without taking the server offline, thus maintaining continuous uptime and performance.

In contrast, other technologies mentioned, such as Plug and Play, refer to automatic device configuration upon connection but do not specifically imply that this can occur while the server is running. Warm site and hot site pertain to disaster recovery strategies, indicating different levels of backup readiness and operation during failure scenarios, but they do not address the ability to add devices dynamically. Hence, hot plug is the most appropriate choice for accommodating hardware changes in active server environments.