Unfortunately, creating a bootable OSX USB install drive isn’t as simple as it should be. I think that we should have than easy way to create a USB drive instead of having to resort to guides like this!
1: Download Yosemite (if it opens the installer afterwards, close it with Command+Q)
2: Format a USB drive
Find a USB drive that is at least 8GB in size Open Disk Utility, select the USB drive on the left & choose Partition on the right side of the window. Select the dropdown and choose 1 partition, set the format to “Mac OS Extended (Journaled)” and set the Name to anything you want. I’m using “Install” Click on the options button below and select the “GUID Partition Table” option & click OK Click Apply and confirm to format the drive
3: Open Terminal.app and run this command (changing the drive name, if you changed it). You will be asked for your password. This command will take a few minutes so don’t interrupt it!
4: Eject the drive and you are ready to install Yosemite!
5: When restarting, hold the ALT key as soon as your Mac powers on to show a list of devices to boot from and choose your newly created drive