Macbook Air 6,2 High Sierra not detecting bootable USB stick
If you have succesfully made a bootable linux USB stck before with your method, try a different USB stick or image. It sounds like you have done this before, which suggests perhaps that the image is not right in some way.

A GUID partition table and FAT format are correct.

Other things to try:

1. Try Etcher: It's simple and free. You can find a tutorial from Ubuntu regarding making a bootable Ubuntu drive with it, but all it really says is "use Etcher."

2. Try Mac Linux USB Loader. It costs ~ $5, and while it only supports certain OSes, Ubuntu is one, so Debian probably works. 

3. Try dd - note it is /dev/diskx not /dev/diskxs3 (partition 3 or other on disk x). Another tutorial also has you convert the image; I haven't used that method but it seems to work for people:

dd if=/path/to/image of=/dev/diskx bs=512k &amp;&amp; sync


I like to know what dd's progress is, so I use pv, which you can get using Homebrew or Macports:

dd if=/path/to/image | pv | dd of=/dev/diskx bs=512k &amp;&amp; sync


You'll probably have to put sudo before each dd command.

I hope that helps - you'll definitely get something to work eventually, whether it's the image, method, or USB stick.