How to Set Up a USB Key to Install CentOS 7
Starting with CentOS 6.5, one can install from USB keys by simply transferring the desired ISO using dd.
dd if=CentOS-6.5-x86_64-bin-DVD1.iso of=/dev/sdb
When asked for the media to install from, select "hard disk" and then the device corresponding to the USB key.
Make sure you select as destination the device corresponding to the USB key (/dev/sdb in the above example) and not a partition (such as /dev/sdb1)