Disk utility won't let me use the restore option to get a .dmg image onto a USB I'm tying to make a bootable Debian USB drive. All I need to do is restore the USB drive from the Debian dmg. I get this error. The debian.org documentation is not helpful with this specific issue. Here's what I keep running into.
I would suggest using dd, from the command line (Terminal).

The syntax is as follows:

sudo dd if=/path/to/file/&lt;file&gt; of=/dev/disk&lt;your drive number&gt; bs=1m

It will prompt you for your password, and you will not be able to see it as you type.

To get your disk number, you can run diskutil list from Terminal, and find the disk that most closely matches your drive's stats.