Apple has now changed its answer to the question "Can external USB, FireWire, or Thunderbolt hard drives be added to Fusion Drive?" found here. The answer is now this -- "An external drive cannot be used as part of a Fusion Drive volume. Fusion Drive is designed to work with an internal hard disk drive and internal flash storage." That seems to confirm what I was told at an Apple Store last week...that you can add an external drive to a Mini using a Fusion Drive...it just can't house the OS. Previously, the answer was very ambiguous...stating simply that you "could not use and external drive with a Fusion Drive." The key change there is Apple stating it cannot be used as part of the Fusion Drive volume.