UCS C200 M2 with ESXi 5.1.0-799733 Image downloaded from MyVMware: ESXi-5.1.0-799733-custom-Cisco-2.1.0.3 ISO mount program: Rufus 2.1.1 vSphere Cliente 5.1.
Can your server successfully mount other ISOs with this particular USB bootmaker? You will want to make sure your C200M2 has a later firmware on it that allows booting to USB.