Hi, iso is for CD img is for USB
so if you want to use USB, all you need is img. (it is not waste your CDs during try and error) see: ftp://ftp.netbsd.org/pub/NetBSD/NetBSD-7.1/amd64/INSTALL.html#Quick%20install%20notes%20for%20the%20impatient blank screen on booting is another story -- Ryosuke
