There are different tools to make a forensic copy of a harddrive. So you have RDD, LinEn. LinEn is a tool written by Guidance to make images on a Linux system using EnCase. But to compare the stuff I did some testing stuff with a USB Memorystick of 512Meg. The results are shown below:
So as you can see… RDD is the fastest one!
Note:RDD is on a BSD-license… LinEn not!