I've tried a few times and have always been unsuccessful. I followed the instructions from quite a few tutorials, and I have been able to get Linux and Windows installed but Grub doesn't detect Windows and just boots into Ubuntu.
The partitions I want to get running are:
SSD 1: Windows (127GB)
SSD 2: EFI Boot (500MB)
HDD 1: Ubuntu Root (56 GB)
HDD 2: Ubuntu Swap (8 GB)
HDD 3: Storage (~ 950 GB)
Try boot-repair. It's a utility tool that will allow you to configure grub and (I'm not sure but I think it also allows you to) select partitions on different hard disks with OS to be added to the grub menu. Google grub repair. I'm pretty sure it'll be helpful.
Choose the standard configuration and it will detect os's . This is the easiest way, but you will get extra entries on your grub menu.
You Should update your Grub loader to see Windows 10 in your Grub Loader Entry. Initially, boot to Ubuntu by normally or Live USB or DVD. Connect your HDD which contains Windows 10. Open Terminal and Run the command
sudo update-grub
Now you can see the list of bootable partitions. like Windows 10(loader). Your Grub is updated restart your computer and now you can see list OSs you can boot into!
It looks like Windows was installed with EFI on GPT partition table, but Ubuntu was installed with legacy MBR partition table and GRUB will not play well in this setup. Reinstall Ubuntu making sure that you boot the USB drive or DVD in UEFI mode and Ubuntu will install with an EFI partition. Then GRUB should automatically detect Windows and give you an option at boot.
