Recently I changed the internal hard drive in my macbook pro with secondary internal hard drive, with this solution from mcetech.com. You can use the secondary drive either to increase the total storage or to act as a RAID drive.
http://www.mcetech.com/optibay/index.htmlI use 500 GB notebook form factor drives for both primary and secondary bringing the total storage to 1TB.
The optical drive even in the mac mini can be replaced with a secondary internal HD.
