Open Terminal or iTerm and run:
sudo /Applications/Install\ macOS\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/USBDrive/ --applicationpath /Applications/Install\ macOS\ Sierra.app
Replacing
/Volumes/USBDrive/
` with the name of your USB drive.
Load Disc Utility, resize and copy the installer files
Once you have created the correct partition you can use createinstallmedia
to copy the install file onto the partiton and *make the partition bootable`
sudo /Applications/Install\ OS\ X\ Mavericks.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ OS\ X\ Mavericks.app