Windows 10 Installer Disc Could Not Be Found

Running Mac Pro and El Capitan 10.11.6

Trying to boot camp Windows 10

I have the ISO on a bootable USB created by Boot Camp Assistant. I have also tried creating one from another Windows 10 box using Microsoft's Media Creation Tool. I've tried re-downloading the official Windows 10 ISOs from Windows, and from Mac.

The intended destination drive is an internal HDD, 1TB, that has nothing else on it, and will be partitioned for nothing but Windows. The other internal drive is a 2TB SSD that's running Mac OS. But I don't even get to a point where Boot Camp prompts me for partitions or drives. In all cases/media types as noted above, when I run Boot Camp Assistant and select "Install Windows 7 or later version", I get an error saying "The installer disc count not be found. Insert your Windows installer disc and wait a few seconds for the disc to be recognized."

Thoughts? Suggestions?