Your bootable USB drive could not be created. An error occurred while copying the Windows installation files.

I spent an hour or so figuring this out. Downloaded the official Windows 8.1 ISO, but the Bootcamp assistant did not want to properly format my USB disk. After finding this http://apple-experience.blogspot.com/2013/08/your-bootable-usb-drive-could-not-be.html I became suspicious that maybe the problem is the ISO itself.

I found this then: http://www.makeuseof.com/tag/how-to-create-windows-compatible-iso-disc-images-in-mac-os-x/  and essentially converted the MS original ISO into a new ISO which was working fine with the Bootcamp assistant.

(I used disk utility to create a .dmg rather than a .cdr by selecting "hybrid image" in the image format drop down)

$ hdiutil makehybrid -iso -joliet -o IR3_CPRA_X64FREV_EN-US_DV9.iso IR3_CPRA_X64FREV_EN-US_DV9.dmg