Use Carbon Copy Cloner--you can clone directly to another partition/drive or make an image and then clone that to the drive you want it on later. Hasn't failed for me yet.
I have used Disk Copy to successfully dupllicate CDs, even Windows install discs, but I had to do a "device copy" (which copies everything at a low level), then burn the resulting image to a CDR.
Whether this would work with an OSX HD or not, I'm not sure, but my offhand guess would be no.