Hello, was using win10 but trying to install win8.1 with cmd, got me into this problem "operating system not found". Tried to install win8.1 with USB pen drive after making USB pen drive bootable but doesn't work. After i press boot menu key in my PC it is f8 key i got 2 options 1.Toshiba (my hard drive) n 2.USB (pen drive) to boot but after i click USB to boot up Black screen with cursor popup. I didn't understand why it shows black screen with cursor.I have asus motherboard m5a78l-mlx3, amd processor 3 gigahzrt, 8gb ram.