Thanks Jake Lo for your reply!
Yes, I'did this process more than once and tried different thumb-drives also.. Yesterday I've tried your MyHack method and had more success as with the Clover method.
My System is booting from USB and stucks with an error: "BootCacheControl: Unable to open /var/db/BootCache.playlist: 2 no such file or directory"
But no reboot!
The Extra-Pack i used might be a problem, because its for Mavericks 10.9.2 and I've downloaded 10.9.4 from the Appstore. Another Point is that I've the FullHD display and not the 1366x768 one as mentioned in the guide.
Thanks for your help!