Creating a Bootable USB drive for a PC on a Macbook
There's a simpler way than above. It's dd.

If you need a new .iso, Microsoft have Windows 10 USB ISOs on their website for download.

Plug the USB device into your mac, and open terminal. 

Type diskutil list and note what the address of your USB is (should be /dev/diskX where X is a number).

If you don't have pv installed, it's worth installing because you get a progress bar for the next bit. If you have Homebrew, then you can just run brew install pv in terminal.

With pv:
type pv -ptearb &lt;full .iso path&gt; | sudo dd of=&lt;your usb address&gt;. You should get a prompt for your password.  

Without pv:
type sudo dd if=&lt;iso path&gt; of=&lt;your usb address&gt;, enter password, and away you go. dd won't show a progress bar though, you need pv for that.

To speed up dd in both cases, you can use /dev/rdiskX rather than /dev/diskX. 

Be careful with dd - if you get the device wrong, you could overwrite your OSX drive