Hi, i update my Mac Pro 2010 from 10.10.3 to 10.10.4 after restart it dosn't boot anymore into osx it just restart after a while. So i want to boot from a USB Boot drive with 10.10.4 on it. But the Drive dosn't show up when i hold alt at start up. The same when i try it with a boot dvd. i have also tried the Drive with my Mac Book Pro Retina there it was no Problem to Boot with the USB Stick. I did a PRAM resett but same result no USB Boot Drive. Now i don't know what i can do to reinstall OSX or repair it.