Windows Home Server Drive Extender lets you add internal or external hard drives of any size, which’ll then create an unlimited amount of storage space for your HD movies, pictures, music, photo etc. WHS Drive Extender provides reliability benefits of RAID (Redundant Array of Independent Disks) without complexity. WHS supports reliability by duplicating designated shared folders – so important data (e.g. your Photos, Music, etc.) will be stored on separate hard drives helping to provide protection against hard drive failure. Folder Duplication’s configurable on a per shared folder basis, so a given shared folder can have multiple copies, with each one being stored on a separate hard drive.
More info: Windows Home Server Drive Extender