Open up disk utility and format your thumb drive as “Mac OS Extended (Journaled)”

Open up a terminal, and run the following command.  Change “USBdrive” to your USB Drives name.

sudo /Path/to/Yosemite.app/Contents/Resources/createinstallmedia --volume /Volumes/USBdrive --applicationpath /Path/to/Yosemite.app --nointeraction

Enter in your password and let it run.

Example:

sudo /Applications/Yosemite.app/Contents/Resources/createinstallmedia --volume /Volumes/OSXInstaller --applicationpath /Applications/Yosemite.app --nointeraction