HOWTO: Write a Disk Image to USB Media on FreeBSD

2021-05-11

This HOWTO will describe the process of writing a disk image to USB media using only tools available in all FreeBSD installations. I wrote this using FreeBSD 13.0 but the command line tools and options should work on older versions.

A disk image will usually end with .img or .iso. A .iso can usually either be written to USB media or to a CD/DVD.

This document will assume the reader has a very basic understanding of the command line.

Steps

Insert the USB drive into the USB port. If using other media, insert the media into the reader/writer and insert that into the USB port.
Open a terminal and run geom disk list to view all attached disks. You should be able to identify it by the size of the disk (“mediasize”) and the manufacturer’s name (“descr”).

Sample output of geom disk list:
```
Geom name: da0
Providers:
1. Name: da0
   Mediasize: 8166703104 (7.6G)
   Sectorsize: 512
   Mode: r0w0e0
   descr: Verbatim micro USB
   ident: 11122488000854
   rotationrate: unknown
   fwsectors: 63
   fwheads: 255
```
The name is what we need. Here that’s da0, but for the remainder of this document I’ll refer to it as XXX to prevent copy/paste accidents.
Run su to switch to root.
Run the following command.

WARNING

This command is destructive. The disk’s data will be entirely overwritten and it will be impossible to retrieve. Only proceed if you’re sure you’re using the correct disk.
```
cat myfile.img /dev/zero | dd of=/dev/XXX bs=1M
```
This command will write myfile.img to the XXX disk, and then continue to write zeros to the disk until the disk is full.

Notes

It’s important to write zeros to the remainder of the disk as sometimes old data can be interpreted as part of the disk image.

The bs=1M flag is necessary because writing a disk byte-by-byte is extremely slow. Writing a disk in 1 mebibyte blocks (1MiB = 1 mebibyte = 1024² bytes) will be significantly faster in most cases. The ideal block size flag to pass will differ depending on the hardware used, but 1M will work well enough. (More about the bs flag)

Steps

WARNING

Notes