As someone who has long had a file server tucked away in a closet, I've always been intrigued by Microsoft's Windows Home Server operating system. The OS's built-in backup and file-sharing functionality aren't all that exciting, but its Drive Extender feature is considerably more interesting. Like RAID, Drive Extender is a redundancy scheme that spreads data over multiple drives. However, rather than dealing with raw data at the drive level, it works with individual files, enabling users to be selective about what's being replicated. Dealing with redundancy at the file level also allows Home Server to easily add new drives to its pool of available storage.
The ability to expand a server's redundant storage capacity by simply plugging in a USB hard drive seems like the kind of feature Microsoft would want to keep around for Home Server. Surprisingly, though, Drive Extender has been dropped from Vail, the next version of the Home Server OS. According to the official Windows Blog, the affordability of high-capacity hard drives has lessened the need for Drive Extender. Also, the Microsoft team responsible for the decision "felt the Drive Extender technology was not meeting our customer needs." Those customers aren't just home users. As a subsequent post explains, Home Server is tightly tied to Windows Small Business Server 2011 and Storage Server 2008 R2, both of which target businesses.
Judging by the comments on both blog posts, Microsoft has a fair number of customers who are more than a little miffed by the decision. I can't say I blame them. Although Drive Extender is the sort of thing I expect most businesses can live without, it's arguably Home Server's most important feature for, you know, home users.