The 2TB 2.5" form factor HDD's are thicker than normal - I believe 15MM instead of the normal 9.5MM because they have an extra platter. You may not have enough room with a second drive in there, it might work with a slim SSD. I recall someone in a previous thread did a measurement of the total space available but I couldn't find it so I can't answer your question for sure. The easier solution would be to keep it external. If you have a new mini with USB 3.0 then the disk will read/write at full speed anyways.