Notes on backing up entire hard disks or partitions

It's often useful to make an image of either an entire hard disk or an entire partition. One reason is to duplicate an installed system onto another PC (probably over a network connection); another is to make a backup of your complete hard disk including every aspect of the installed operating systems, which you can restore if you have to replace your hard disk or if you screw things up. Typically it's useful to be able to transfer these images over the network to another machine, although you may want to save images onto a different partition or hard disk.