i used the same custom made cable adapter as for the early 2009 mini (something like this http://shop.ebay.de/?_from=R40&_trksid=m...). i replaced the 12V female plug with a sata power male plug, and voila, you can put the sata and power plug into your 2nd harddrive and the slim sata connector to the flexbandcable that used to connect to your superdrive. the hard part is putting the cables in the right place so you can squeeze in the logic board where it belongs :-P
works like a charm in my 2010 mini with 2x500 gb, even though i will buy a 1TB 2.5" drive and a 60 ggb ocz vertigo 2 ssd as system drive :-)
ps: i just drilled two small holes into the dvd caddy and used two screws to attach the hard drive!