I have an old motherboard that requires creating a DOS boot floppy in order to upgrade its BIOS. Fortunately, it's not too hard to do this with FreeDOS and a USB stick.

The instructions below are based on an old FDos wiki article. You maye have more luck with the latest instructions from the official wiki

Downloading the dependencies

The first step is to download the required files from your motherboard manufacturer:

• the latest BIOS image
• the BIOS flashing program

and then install the tools you'll need:

apt-get install makebootfat syslinux

Preparing the "floppy" image

Start by collecting all of the files you need to install FreeDOS on the USB stick:

cd /tmp

wget https://www.ibiblio.org/pub/micro/pc-stuff/freedos/files/distributions/1.0/pkgs/commandx.zip
wget https://www.ibiblio.org/pub/micro/pc-stuff/freedos/files/distributions/1.0/pkgs/kernels.zip
wget https://www.ibiblio.org/pub/micro/pc-stuff/freedos/files/distributions/1.0/pkgs/substx.zip
wget https://www.ibiblio.org/pub/micro/pc-stuff/freedos/files/distributions/1.0/pkgs/unstablx.zip

for ZIP in *.zip; do unzip $ZIP; done

cp ./source/ukernel/boot/fat16.bin  .
cp ./source/ukernel/boot/fat12.bin .
cp ./source/ukernel/boot/fat32lba.bin .

cp /usr/lib/syslinux/mbr.bin .

and then create a directory for the files that will end up in the root directory of the "floppy":

mkdir /tmp/fs-root
cp ./bin/command.com /tmp/fs-root/
cp ./bin/kernel.sys  /tmp/fs-root/

and copy the BIOS image and update program into that same directory (/tmp/fs-root/).

Creating a bootable USB stick

Plug in a FAT-formatted USB stick and look for the device it uses (/dev/sdb in the example below).

Finally, run makebootfat:

/usr/bin/makebootfat -o /dev/sdb -E 255 -1 fat12.bin -2 fat16.bin -3 fat32lba.bin -m mbr.bin /tmp/fs-root