I keep trying to use an 8GB SanDisk drive to boot the OS on a PC, and no matter which version I use of 5R1, the PC's display still in the DOS screen "Missing Operating System." What does this mean? I create the UNETbootin on a MacBook Pro, maybe 2 years old. I've tried both GNOME and KDE (which shouldn't really make a difference), and neither have worked. The PC has an AMD 64 bit processor