Mac Disk Utility

[asterroc]

I'm trying to use Disk Utility to make a disk image of the whole computer on my brand spanking new external HD for backup purposes. However, when I click "New Disk Image" or any version of that thru the File Menu, it says (roughly) "Unable to create image of ****. Resource in use." This sounds to me like it won't do an image of the same disk that Disk Utility is hosted on. So how the fuck do I backup my whole computer?