How to use OSX to create a bootable USB key to install Windows 8.1 on a PC
Quick shot:


Download and install VirtualBox
Create a new Windows VM with your Windows 8.1.iso (x64)
Copy the Windows 8.1.iso (x64) to your VM (network sharing)
Download and install Rufus
Plug in your thumb drive and attach it to the VM
Create a bootable thumb drive with Rufus. You may even choose UEFI or BIOS.


Not smart but it should work...

I'm searching for a faster way without creating a VM