Mac Mini mid 2010 drive identification
I've no idea of the construction of a Mini, but you could test it empirically.
Remove one drive, see if it will boot.
If yes, look at Disk Utility & see which drive remains.
If no, then you removed the only boot source.
After removing both internal drive I determined that the second hdd is underneath. So everything has to be removed to get to it. This includes fan, system board, power supply and brackets. A very good detailed instructions can be found at https://www.ifixit.com/Guide/Installing+Mac+Mini+Mid+2010+Dual+Hard+Drive/8958.