I have a DVD with fedora 20 in it. My DVD drive doesn't work so I need to boot from an USB. I have copied the contents of the DVD into a USB drive and now how can I make this drive bootable?
If the drive is able to be booted at all, that is if you successfully copied the right data to it, then you shouldn't need to do anything other than to actually boot to it (from the BIOS or UEFI as the case may be).
Just copying the DVD contents over is not enough to make the USB stick a bootable device. You need to run it through an App like UNetbootin1. It will even download the ISO image for you or you can also provide your own.
You can do it simply with the dd command:
dd if=/dev/usb_key of=/pyth/to/mydvd.iso