mac pro (early 2009) failed to boot after upgrading EI Capitan

Previously the 2009 mac pro was running OS X 10.10 correctly. During upgrading it to the newer 10.11 EI Capitan, the machine restarted and can't boot anymore, staying at the stage with the whole white screen (without Apple Logo showed). After that, I tried installing a new clean EI Capitan from a bootable U drive (press Option key and choose that U drive disk to boot from), in this case, I can see that U drive disk is recognized by the system, but when I tried to double click the icon to start booting from the U drive and install the new OS, nothing happens. I'm totally sure that the bootable U drive is correctly created and tested in a MacBook Air.

In this case, how can I install a clean OS to this machine??? I have been puzzled by this problem for quite a long time. Any information would be appreciated.

Thanks.