actually it does void warranty unless an authorized apple repair shop does it. there is a but and here is the but.
if you do it on your own and don't break anything no one can tell if you set the mini back to stock . then bring the mini in for repair. If you bring a 1 hdd mini in for repair that has a second hdd in side of it you will be told the mini is not under warranty.
Frankly It is no longer worth doing.
I use an external t-bolt drive easy to do and no warranty risk.