upgrade OS.. Centos 7

In the wasted time I finally installed last Centos OS

I used the usb support in order to install. The upgrade from previous version isn't possible: I have installed the 5.x version.
New fresh installation from USB device was necessary.. and before a full backup of my personal-server.
It was very difficult the installation because error "/dev/root not exist.. like this
topic

# cat /etc/centos-release
CentOS Linux release 7.0.1406 (Core)