Backup a Mac without the OS
This is not hard and all you need is an external drive the same size as the drive you intend to copy.
Boot your Mac from a bootable Linux CD/DVD distro, there are a number of them. Plug in second drive and learn to use the shell command dd. You then have an exact copy of the drive. Mac volumes may also be mounted on most Linux distros without adding any software if you need to inspect the data.
I run a demo centre have done this often over the last ten years on many different machines and architectures.