Connect an external USB drive or insert a flash drive into the USB port on your Mac; keep it connected or inserted while you install Windows and the Windows support software. The system is checked for total available disk space. This process may take a long time to complete you can click the Stop button to skip this process. The Windows files are copied to the USB drive.
This process may take a long time to complete you can click the Stop button to interrupt this process. At the Create a Partition for Windows step, specify a partition size by dragging the divider between the macOS and Windows partitions. If you have multiple internal hard drives, you can select a different hard drive from the one running macOS and create a single partition on that drive to use solely for Windows.
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Doing so may delete the entire contents of your macOS partition. Important: Do not click the Cancel button in any of the installer dialogs. If nothing appears to be happening, there may be a hidden window that you must respond to. Look behind open windows.
Once you have that completed, make sure that you have installed Toast. Make sure that both are checked, and then proceed to the next screen. You can not save these files to the bootable USB without some prior customization, so I suggest a second USB drive or just saving them to a folder on your desktop. Fortunately you can do this yourself during the installation process.
Install 64-bit Windows on your macOS computer using Boot Camp Assistant
This does not appear to have any ill effect on your OS X installation, but please make sure you have a current backup in place. On many occasions I had to wait 3 or 4 minutes on these screens before the installer would continue. Be patient. On a couple of occasions it appeared that no master boot record was written to that drive, as opposed to the main drive where it worked every time.
The problem here seems to lie in having more than one drive active in the system. The answer? Disconnect one of the drives.