Quick tip if you encounter problems in the partition dialog of Windows 10 when trying to install it via Boot Camp on a Mac running High Sierra.
Two problems cropped up for me. The first and simplest was the Boot Camp Assistant doesn’t format the partition as NTFS. No big deal – click it (it’s named BOOTCAMP by default), double-check that you’ve selected that partition, triple-check because this is dangerous, then click Format.
But once that was done, the Windows installer complained with a message like “We couldn’t create a new partition or locate an existing one. For more information see the setup.log files.” After a lot of searching I found a suggestion to unplug other connected drives before trying the install.
And that did the trick. Right then, with the partition dialog up, I unplugged my USB backup drive and tried again. The install proceeded normally after that. Note – I did _not_ unplug the USB stick with the Windows install on it. Probably obvious, but figured worth mentioning for completeness.
Now here’s hoping tagging this properly helps someone else find this answer faster than I did…