I have a Windows 10 machine installed on an SSD, installed another hard disk and I installed Ubuntu 15.10 on it. I didn't specified anything when installing, but now it boots directly into Ubuntu. It doesn't give me the option to select which OS to boot.
First try to go into your Terminal Ctrl+Alt+T.
Next type in:
wait for that to finish, then do:
Finally restart your computer and see if the windows 10 shows up in the boot menu, it might say windows 8 loader which is what mine says.If that doesn't work I will look for something else.
You have one of two options, here they are:
Option 1. Type Ctrl+Alt+T
and type in the following command
once that finishes do:
now restart your computer and you should see a list of operating systems..
Choose whichever you prefer / want.
Option 2. Power off, and then turn on your computer..
While it is turning on press the f12 button on your keyboard.
You should see a list where you can select windows (using your arrow keys)!
Enter the boot menu when you're at POST screen and choose your SSD, it should boot into Windows 10 as long as you didn't modify the MBR. You can as well edit the default boot drive in your BIOS.