FreeDOS Flash Drive

Almost every BIOS/firmware update needs DOS to work, this article aims to explain how to create a USB flash drive, used to boot on FreeDOS and run those medieval programs. (You will need to have a motherboard BIOS capable of booting from a USB device. Some may also need to enable this option from within their motherboard's BIOS setup screen as well.)

Requirements
There are several packages you'll need for this,, which provides various bootloaders (Syslinux, PXELinux, etc.), , The GNU GRUB 2 boot loader, which provides ( is part of @system, so you already have this) and last,  which provides the DOS filesystem tools( etc.).

Emerge these packages before continuing:

Partitioning
First thing you need to do is create a partition, let's assume the USB flash drive is :

Remove everything, create a primary partition, set its type to 0E(W95 FAT16 (LBA)), mark it bootable, write the partition table and quit.

Formating
Now you can format the partition in FAT16, you can try FAT32 but FAT16 is recommended :

FreeDOS
There is a nice image of FreeDOS with all you need, you just have to copy a file from it to the partition:

MBR
Next thing you need to do is make the USB flash drive boot on something, that would be GRUB. You will also need a file from Syslinux to boot FreeDOS.

A nice configuration for GRUB :

And finally, you can install GRUB :

Last touch
Now you can copy your BIOS update/whatever to the USB flash drive.

Last step:

That's it, you can boot on your USB flash drive. Once you are in FreeDOS, your flash drive will be, enjoy...