I'm going to add to the chorus of Windows 10 external SSD posts, I'm sorry. But my position seems a bit different from most. I've installed Windows 10 on an external T3 ssd, connected to my 2017 iMac. If I hold down option when I power it up, I can select "EFI Boot" and load Windows 10 and it works fine. But, annoyingly, I can't select this in Startup Disk utility. A "Bootcamp" Windows drive shows up, but the bless tool won't let me select it. And when I go to disk utility it says there's a problem with the partition map. The think is, Windows runs totally fine and I can boot it holding down option. So I can ignore these issues, but I'd love to know if there's some way I can fix the partition map or make it work from Startup Disk, which I find a lot more convenient.