I successfully installed Windows 10 Pro on an external SSD connected to a USB3 port. This was done using WintoUSB software that mimics Windows to Go. I could then connect the drive to either of two PCs and boot from it. I then installed Elementary OS Loki onto the same drive. To do so I created three partitions in the unused space after the Windows NTFS partition. On installation I chose "Something Else" and made the three partitions I had created into "Root" "Linux Swap" and "Home" partitions and completed the installation. On booting up the computer after installation the Grub screen appeared with only two options, to boot into Elementary, or Elementary advanced options, and with no option to boot into Windows. Windows is still all there on the drive. I can see it by plugging the drive into a Windows PC, but I just cannot boot into it. What Have I done wrong? Can it be fixed, or do I have to start again?
I do not know if it will work, but you can try to type into the elementaryOS terminal
sudo update-grub and check out do you have a message similar to
Found Windows Boot Manager on /dev/...
This is the official way to add boot entries so if it founds it probably will add it like an option. I type 'probably' because I have no experience with dual external ssd boot. Hope that helps