Drives, fans, and power supplies are server components that can most commonly be replaced without downtime if they are hot-swappable. Hot-swappable components can be removed and inserted while the server is running, without affecting its operation or performance. Drives store data and applications, fans cool down the server components, and power supplies provide electricity to the server. Replacing these components can prevent data loss, overheating, or power failure. References: CompTIA Server+ Certification Exam Objectives, Domain 2.0: Hardware, Objective 2.2: Given a scenario, install, configure and maintain server components.