I decided I wanted to dual-boot Linux/Win10, each to their own SSD. This, mostly, went off without a hitch. But I'm really tired of having to change the boot order to change OS.
After reading around, I'm pretty sure the issue is that I installed Linux (Peppermint 8) in Legacy and Win10 is in UEFI.
I've tried running Boot-Repair, but it complains it's in a Legacy session and I need to boot it in a UEFI session, which I don't know how to do. I tried a BIOS option of 'Boot USB in EFI Shell' but that didn't seem to work.
Honestly, if there's a way to put Win10 into Legacy mode easily, I'd accept that too.