How to mirror a harddisk for backup purpose

[Lewin, A]

<snip>
>The problem with tar is that you need disk space to store the
>intermediate
>file between your backup and restore.

It's a while since I've done it (so I've probably forgotten the syntax)but
I'm fairly sure it works ok if you mount the second HD somwhere and:
tar tar c / | tar x -C /2nd_hd_mount_point
without creating any files anywhere.

I think you can do this from one machine to another as well using rsh
something like:
tar c /|rsh remote_machine 'tar x -C /mount_point'

I remember writing a script to auto-clone about 30 identical nodes (all
initially booted using a floppies) using rsh, it went something like:

rsh remote_machine mount hd partitions in correct tree formation on /mnt
tar c / | rsh rsh remote_machine tar x -C /mnt
rsh rsh remote_machine chroot /mnt /sbin/lilo
some cunning bits to set the IP addresses etc (writing direct to the config
files)
rsh rsh remote_machine reboot

looped through the machines one by one overnight!