> 
> **Possible Duplicate:**  
> 
> [What makes bootable media bootable?](https://superuser.com/questions/223664/what-makes-bootable-media-bootable) 
> 
> 
> 

As far as I believe, if you 'drag and drop' an ISO image onto a device such as a usb or cd it won't boot as It's just random files, the BIOS doesn't know It's bootable.

What makes a device bootable to the BIOS; how do you flag a device bootable? What files are essential to a bootable device, ie .img files?