Windows 8 will include a new virtual disk feature to enable Windows Home Server-like functionality. Storage Spaces lets Windows 8 users group together physical disks into a larger storage pool for increased resiliency in the case of a physical failure in other words, USB, SATA, and SAS disks are all collated into one virtual disk. Storage Spaces appears to build on a similar functionality called Drive Extender, formerly in Windows Home Server.
Virtual disks look like regular physical disks, and users can partition, format, and copy data to the drive much like traditional storage. With thin provisioning, the process behind this magic, users will be able to create a virtual drive of any size. So, for example, you can create a 10TB Storage Space, even though you only have a 500GB and 1TB drive. While you'll bump up against the 1.5TB limit, Storage Spaces's built-in resiliency comes from having at least two copies of all data, within the virtual disk, on at least two physical disks.
If a physical disk fails then it can simply be replaced with little effort, like a traditional RAID setup.
Windows 8 is set to enter beta in late February.
[Source: Building Windows 8, TheVerge]