I'm familiar with the fact that my model has space for a second internal 2 1/2 HD, and also has a second SATA connector on the mother board. My choice is to either install the new SSD as a second internal HD in the mini, or to pull the old conventional drive and retire it to an external enclosure, leaving the new SSD as the sole internal HD in my mini. (Macmini 5,2) I'm planning to upgrade to a Solid State Drive (SSD). I have a mid 2011 Mac mini, 2.7 MHz, i7, 8 GB Ram with the 7200 RPM Western Digital 750 GB factory upgrade drive.
