To put it simple. I have opened the laptop to install the SSD i'm currently using. Since I won't have power to feed a HDD connected to the M2 with only SATA, I'll install the HDD to the slot with power and the SSD will be connected only through the SATA. And one more time, I've opened the laptop and I KNOW that there is enough space. Look at the older models that have a CD/DVD to see how much space it takes, then take the CD/DVD out and you will see EXACTLY how much space I have!
Laptop for reference: Acer Aspire 5750G which is a little smaller (by a little I mean < 1'').