Here is how to make a bootable usb of the yosemite installer after you download the full installer app from the mac app store.
Format an 8 GB USB drive which should be called Untitled and formatted as Mac OS Extended (Journaled). The installer should be called Install OS X Yosemite.app and should be in your Applications folder.
Run this in terminal and wait about 20 minutes:
sudo /Applications/Install\ OS\ X\ Yosemite.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath /Applications/Install\ OS\ X\ Yosemite.app --nointeraction
You can boot up from it by selecting it from the startup manager you get when starting your computer and holding down the option key.
Format an 8 GB USB drive which should be called Untitled and formatted as Mac OS Extended (Journaled). The installer should be called Install OS X Yosemite.app and should be in your Applications folder.
Run this in terminal and wait about 20 minutes:
sudo /Applications/Install\ OS\ X\ Yosemite.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath /Applications/Install\ OS\ X\ Yosemite.app --nointeraction
You can boot up from it by selecting it from the startup manager you get when starting your computer and holding down the option key.
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