disk utility will not create new image of hard disk

I have tried to create a new image onto an external hard drive using disk utility. I have tried this in recovery mode and also when logged in using the external HD with the reinstalled system software as the boot disk. I get a permissions issued when trying to copy the Macintosh HD to the external HD.

I have tried using terminal, but my UNIX is terrible.

Time machine is a no no because I cannot see the external HD as an option to back up to.

Disk utility when I am logged in doesn't work, disk utility in recovery mode doesn't work, I don't have an option to use firewire transfer as I don't have another computer to back up to with firewire capability.

Does anyone have any suggestions that I haven't tried?

Yours in frustration

Sandra