How to burn Windows 10 ISO to USB?
First, by "flash drive" we both understand external USB flash drive, right? :) If not, then get one :) Make sure it does not contain any important information.


Insert clean USB flash drive into your mac
Start standard Disk Utility
Select your USB flash drive on the left panel
Click 'Restore' button in Disk Utility window
In the dialog window click 'image' button and select your ISO image
From drop-down list in the same dialog window select your USB flash drive
Press 'Restore'


You should get properly prepared Windows 10 USB installation disk.

Hope this helps.