Mac mini Mid 2011 has an additional SSD BTO. You can have an additional 2.5 inch SSD/HDD.
Mac mini Mid 2011 has a mounter for the 2nd drive, but you need a special cable and some other parts to add the drive. Supports SATA 3.0
1. SATA cable for additional drive (1 pc)
2. Tape to fix the cable (1 pc)
3. Grommets and screws (4 pcs)
Installation is not very easy. Intermidiate skills are required. You can see the online manual here.