The hard drives in the server are hot-pluggable, meaning that the drives can be removed and inserted while the server is powered on.
Depending on the configuration of the data on a particular drive, the drive might also be removable while the server is online. However, to hot-plug a drive while the server is online you must take the drive offline before you can safely remove it. Taking a drive offline prevents any applications from accessing it, and removes the logical software links to it.
The following situations inhibit your ability to hot-plug a drive:
If the drive contains the operating system, and the operating system is not mirrored on another drive.
If the drive cannot be logically isolated from the online operations of the server.
If either of these conditions apply to the drive being serviced, you must take the server offline (shut down the operating system) before you replace the drive.