De acordo com Centos-wiki :
UNetbootin is probably the most popular "outside" method, has both Windows and Linux executables, and allows you to create bootable Live USB drives for a variety of Linux distributions. This utility is no longer recommended for use as it fails to work with newer CentOS iso images.
A solução:
Create a bootable USB key simply by installing the ISO file on the key using a program such as Win32 Disk Imager.