15.6" Defiance II hard drive bays

I bought a 15.6" Defiance II from PCS during summer 2015, and only ordered it with one hard drive, I did note that the fact that the laptop has support for 2 hard drives (Excluding M.2)

Now as we are getting towards Christmas I want to purchase an SSD for my laptop without removing the original hard drive, yet when I open up my laptop I see only the in use hard drive bay and connector, am I not looking in the right place here or is there really only one hard drive bay and connector?

Is it possible that the external SATA / USB3 combo port is the "2nd hard drive that I noted when ordering my laptop?, I highly doubt it seeing as this is external and not internal but it's all I can come up with.

I think the hard drive bay can hold 2 hard drives

I mean the two bays are stacked together,one on the other top

Defiance has 2 x 7mm, 2.5” S-ATA HDD support
If you have a 3.5" mechanical drive, you wont be able to get another hdd or ssd inside.
Why not get a m.2 drive?

I have heard of people having heat production problems with M.2 drives, some users have even had to place miniature heatsyncs on their M.2 drives to keep them cool, I don't want this kind of heat being produced in my laptop