On servers, the front bay of HDD's (or whatever other kinds of drives) are usually designed to be hot-swappable. Meaning, there is not any critical data stored on them. Often there is temporary files or long term storage files on these drives. It's hard to say for sure.
The drive that the OS is installed on is often buried away inside the server so it cannot be mistakenly removed during operation. That would be bad juju.
So yeah, often there is a separate drive inside holding the contents of the OS and operation-cricital files. Other drives are purely for storage. (usually, because any drive can be configured however you want, and I don't know what the last guy did...)