How to Create a Bootable DOS USB Drive
 

DOS isn’t widely used anymore, but you wouldn’t know if from reading instructions written by manufacturers for BIOS updates, firmware-updating utilities, and other low-level system tools. They will often require you to boot into DOS and run the utility.

We once formatted our floppy disks with MS-DOS using the format utility built into Windows, but most computers don’t have floppy disk drives anymore. They may not even have optical disc drives!
Use Rufus

Windows won’t allow you to select the “Create an MS-DOS startup disk” option when formatting a USB drive – it’s grayed out. We will be using Rufus instead. It’s a fast, lightweight application that includes FreeDOS.

Download Rufus and launch it. Rufus doesn’t require any installation – you will see the Rufus application as soon as you launch the downloaded .exe file.

http://www.howtogeek.com/136987/how-...dos-usb-drive/