When you add a hard disk to a virtual machine, you can create a new virtual disk, add an existing virtual disk, or add a mapped SAN LUN.

You can create a virtual hard disk and add it to a virtual machine.

You can add an existing hard disk to a virtual machine.

You can use a raw device mapping (RDM) to store virtual machine data directly on a SAN LUN, instead of storing it in a virtual disk file.