A new incarnation of Windows Home Server's old (and discontinued) Drive Extender technology, Storage Spaces maintains up-to-the-second mirrored copies of all your data, as long as you have two or more hard disks. If one of the disks should die, the data's still available, with nary a hiccup. Remove the dead disk, stick in a new disk that's at least as big as the one that died, wait an hour or so, and the mirrored backup continues -- and you don't have to touch a thing.
All the disks, internal or external, use one single drive letter (in Microsoft parlance, they're "virtualized"). If you run out of space, feed Storage Spaces a new disk, and it gets absorbed into the hard-disk Borg, using the same drive letter as all the others. If you have a handful of old 250GB disks hanging around that you really don't want to throw away, you can stick them in a storage pool and treat them as one big D: drive. If you still run out of space, add a new 2TB disk, and they all play happily together.
Data gets juggled and shuffled behind the scenes, with absolutely no need to copy, move, or otherwise mangle your data. No, it isn't RAID. No fancy hardware involved: Regular, old everyday hard disks, both internal and external, all work together.
It's the way hard disk storage should've been implemented decades ago. It works brilliantly. And Windows 7 needs the feature badly.
Windows to Go
As you probably know, Windows to Go lets you put Windows on a stick. Take the USB stick with you, put it in any Windows PC running any modern version of Windows, then boot from USB -- congrats, you can run a completely isolated copy of Windows 8. Nothing on the host computer goes onto the USB-spawned copy of Win8. Nothing from the USB version of Win8 leaks onto the host computer. You can boot a Windows to Go USB drive on the dirtiest, most infected, and irretrievably bug-laden PC in the world, and the environment you work in remains pristine -- unless you gum it up, of course.
The current version of Windows to Go ships only with the Enterprise version of Windows 8, and it only creates USB-based portable Windows 8 machines. In theory, that's so businesses can manage the USB-based Windows 8 environment for employees who may be working at home or on the road. The Windows to Go image is licensed to the Enterprise account, not the individual. In practice, Windows to Go could be a boon to every Windows user, corporate or otherwise.
Right now, Windows to Go is a "Microsoft version 1.0" product in the pejorative sense of the term. It doesn't play well with networks, and it positively annihilates homegroups. It's slow as molasses on anything but USB 3.0 systems. It requires a 32GB USB drive, as a minimum, and that leaves little room for storage (which admittedly is supposed to go into Microsoft's SkyDrive cloud storage service anyway).
But if Microsoft could build a Windows 7.8 version of Windows to Go that creates Win7.8 machines on USB drives and offer Windows to Go on every version of Win7.8 from Home Premium up, I'd be first in line to grab it.