I wanted to completely reformat and reinstall Yosemite on an old iMac. To do this, I needed to make something I could boot from. I got an 8gb usb flash drive and used Disk Utility to reformat it to a Mac filesystem that I called Yosemite. This disk was then mounted at /Volumes/Yosemite. I then went to the App Store and downloaded Yosemite, but cancelled the installation. This left the installer in /Applications/Install OS X Yosemite.app. Then, to make the bootable drive, I ran the following command:
yo:~ $ sudo /Applications/Install\ OS\ X\ Yosemite.app/Contents/Resources/createinstallmedia --volume /Volumes/Yosemite --applicationpath /Applications/Install\ OS\ X\ Yosemite.app --nointeraction Password: Erasing Disk: 0%... 10%... 20%... 30%...100%... Copying installer files to disk... Copy complete. Making disk bootable... Copying boot files... Copy complete. Done. yo:~ $
I don’t remember how long it took, but I definitely had to wait a while. I’d say maybe 30 minutes or so. But it worked perfectly.