nixpkgs/nixos/doc/manual/installation/installing-usb.xml

<section xmlns="http://docbook.org/ns/docbook"
         xmlns:xlink="http://www.w3.org/1999/xlink"
         xmlns:xi="http://www.w3.org/2001/XInclude"
         version="5.0"
         xml:id="sec-booting-from-usb">
 <title>Booting from a USB Drive</title>

 <para>
  For systems without CD drive, the NixOS live CD can be booted from a USB stick. You can use the <command>dd</command> utility to write the image: <command>dd if=<replaceable>path-to-image</replaceable> of=<replaceable>/dev/sdX</replaceable></command>. Be careful about specifying the correct drive; you can use the <command>lsblk</command> command to get a list of block devices.
  <note>
   <title>On macOS</title>
   <para>
<screen>
<prompt>$ </prompt>diskutil list
[..]
/dev/diskN (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
[..]
<prompt>$ </prompt>diskutil unmountDisk diskN
Unmount of all volumes on diskN was successful
<prompt>$ </prompt>sudo dd if=nix.iso of=/dev/rdiskN
</screen>
    Using the 'raw' <command>rdiskN</command> device instead of <command>diskN</command> completes in minutes instead of hours. After <command>dd</command> completes, a GUI dialog "The disk you inserted was not readable by this computer" will pop up, which can be ignored.
   </para>
  </note>
 </para>

 <para>
  The <command>dd</command> utility will write the image verbatim to the drive, making it the recommended option for both UEFI and non-UEFI installations.
 </para>
</section>