For OS X Maverick
Get a list of drives on your computer
diskutil list
Unmount the appropriate drive
diskutil unmountDisk /dev/install the ISO Image on>
Copy file to the drive you want to mount the isoimage on
dd if=<path of ISO Image> of=<path of your USB Drive>
For macOS Sierra
sudo /Applications/Install\ macOS\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath /Applications/Install\ macOS\ Sierra.app
Thank You