I've installed Windows 10 on an external USB 3.0 HDD. Currently, the only way I can boot it is by going into my BIOS boot options and choosing it from the list.
I've installed Ubuntu 16.04 on an internal M.2 SSD, but the Ubuntu installer didn't automatically add an option for GRUB2 to boot the external drive like I expected.
I'm not sure how to add a menu entry which tells grub to boot from the external drive which has Windows 10 installed. I know I can use the search command with the external drive's uuid to set the root variable, but I'm not sure how to use the boot command after that, or which file to edit to add boot options.
UPDATE: I ended up doing something like the accepted answer, but much simpler
menuentry "Windows 10" {
set root=(hd0,msdos1)
chainloader /EFI/Microsoft/Boot/bootmgfw.efi
}