dd, sometimes called GNUdd, is the oldest of the imaging tools currently in use. A command line program that has been ported to many operating systems, dd uses a complex series of flags to allow the user to image or write data from and to raw image files. There are a few forks of dd for forensic purposes including dcfldd, dccidd, a Microsoft Windows version that supports reading physical memory.
Here is a common dd command:
dd if=/dev/hda of=mybigfile.img bs=65536 conv=noerror,sync
Use extreme care when typing the command line for this program. Reversing the if and of flags will cause the computer to erase your evidence!