What you need:
- USB drive greater than 16 MB
- FreeDOS Base CD
- Blank CD or CD-RW
- Format the USB drive as a MBR/FAT volume
- Burn the FreeDOS Base CD
- Shut down OS X and disconnect all disk drives
- Boot computer while ensuring that Legacy USB support is enabled in the BIOS and USB drive is connected
- Boot from FreeDOS Base CD and follow instructions to install. It will install by default to the 1st drive it finds. In this case it’s the USB drive as it’s the only one connected.
- Shutdown and reconnect all disk drives.
- You now have a bootable “DOS” drive.