I will try to keep this concise.
I have Ubuntu 16.04 and Windows 10 duel booting on both my laptop and my desktop. I have a bootable USB drive with the Ubuntu ISO on it and used that both times for my two previous installs.
On my desktop when I clicked "Install alongside Windows boot manager" the screen came up that allows me to partition the hard drive. There was a drop down menu that let me choose whether I wanted to install on the 120GB SSD or on the 1TB HDD, and a slider to choose the amount of memory to partition. I chose my HDD and everything works just fine.
I just tried to do the same for my brother on his desktop and for some reason it is not seeing his 1TB HDD. We have exactly the same computer. I have spent a ton of time so far trying to figure this out and I do not understand why It let me install on my HDD but not his. Any help will be greatly appreciated.
P.S. If you need more information please let me know.