I am new to Raspberry Pi. I have just created a dual boot option on my laptop to boot the Raspbian desktop OS. I followed the instructions but when rebooted I had lost my original Win10IOT OS. However, after searching online came across an ask Ubuntu discussion on the issue.
sudo grub-install /dev/sda
This has located the win10IOT again and put it on the boot option.
I am kind of answering my own problem in case anyone else experiences the same, or someone has anything to contribute.