Yeah, to dual boot Windows 10 and something else Secure Boot needs to be off. UEFI can probably be on or off, but I think it's easier to have it on.
I ended up reading various guides which talked about having to copy boot sectors and all sorts, but thankfully I didn't need to do any of that. It was actually fairly trivial once I thought about it properly.
- Have only the empty SSD plugged in so the OS setups can't see any other OSes and get confused
- Disable Legacy Boot in BIOS (or definitely don't pick it)
- Run Win10 setup from USB media - making sure to pick UEFI boot option
- Partition drive in Win10 setup - I split it in half, let Win10 install on the first partition - it split that into boot and reserved bits itself as usual.
- Install Win10 - check it works
- Run Ubuntu (or whatever) setup from USB - UEFI boot
- Hopefully that sees you have a Windows installation and uses the extra unused unformatted partition
- Install 2nd OS
- Ubuntu 17.10 will sort the dual boot bootloader for you
- Reboot and pick Ubuntu or Windows Bootloader from the Grub screen
It currently has a 10s wait on boot, might reduce it since 10s feels like an age!