Bootable devices are needed to install an operating system or to run different utilities for a system. If the USB must be bootable, then you must create a bootable USB stick with your ISO file. There are some easy to use programs that will create bootable devices.
Method 1: Rufus
Open Rufus and make sure to select the USB stick. This is important because all data from that device will be formatted (erased). I like Rufus because detect automatically the USB device.
Then select the ISO file from your hard drive.
Then click Start and click Ok on dialog boxes. Then wait until the USB is created.