If you're looking for a hard drive for a 2010-or-later Mac mini, NewerTech's aluminum miniStack ($79 to $1130, depending on storage options) is a perfect mate. Thanks to identical footprints, the miniStack and the Mac mini stack perfectly, with the two connecting via FireWire 800/400, USB 3.0/2.0, or eSATA. (FireWire and USB cables are included.) You can even pile multiple miniStacks on top of one another and daisy-chain them together using FireWire.
NewerTech sells the miniStack empty for DIY-ers, or you can order it with a traditional hard drive (500GB to 4TB) or a speedy solid-state drive (SSD, 120GB to 960GB). The company also sells the miniStack Classic for pre-2010 Mac mini models.