Originally Posted by Citra
Necessary? No Awesome? Yes.
BTW are you running RAID or JBOD?
Here you go...
Windows Home Server Drive Extender is a specialized case of JBOD RAID 1 implemented at the file system level. When a file is to be duplicated, a pointer called a tombstone is created on the main storage drive's NTFS partition that points to data residing on other disk(s). When the system is idle, the OS re-balances the storage to provide redundancy while maximizing storage capacity. Although not as robust as true RAID, it provides many of RAID's benefits, including a single hierarchical view of the file system regardless of physical disk location, the ability to swap failed disks without losing redundant data, and seamless background data duplication on the replacement disk.
It is also possible to limit data duplication on a per-share basis. Drive Extender can store files on different disks and use tombstones to point to them, providing faster read access when the end-user requests files located on different disks, similar to RAID 0's speed benefit.