Careful what you say, the 2012 mini's were shipped with the HD mounted in either the upper or the lower bays, I do not know Apples reason for this, but I do know mine came with the HD in the lower bay, screwed right to the wi/fi antenna grill.Nice thing with the 2012s is that the HD is in the hard to access position, leaving the easy access space void.
[[ I only got one SATA harddisk for mac mini,is it possible to remove it and replace with SSD? ]]
There's no reason to "replace" the drive, because the Mini has the room and connections for TWO drives.
You want to "add", not "replace"...
You will need another SATA ribbon cable.
I don't agree, perhaps they want the speed of an SSD and don't need the extra internal storage or want the speed of an SSD and are taken back by having to pull out the logic board on a data doubler install. You can get a decent sized SSD like a 480GB M500 for around $240-$250. Seems like a worthy enough replacement to me.