Pretty much what MikeyMikec said except dont get scared by "partitioning", not sure if you are but I was when I first heard it.
After installing windows 10 on the 100gb drive
Turn off the computer, install the second drive and boot back up. - Only thing to make sure here is that your boot drive in bios is set to the 100gb drive.
Then boot into windows, you are now going to look for "Disk Management" in the "Computer Management" program.
A quick way to get to this is to press the windows key and then x and you will have a pop up menu, one of the options should be "computer management", select it and then select "Disk Management"
In Disk Management, just right clicking on the drive should give you a bunch of options, one of which should be "new volume", select the options you want and it will partition and format all on its own and you are done.