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This is useful to keep identical copies of your data on a 2nd disk.  It's NOT a replacement for using RAID1 mirroring – but it can be useful.  e.g. you can access data from the previous backup on a per-file basis.  You could also use it to mirror a current disk to a new disk to go into a seperate server (disk cloning).

What it is REALLY useful for is to copy your data from a smaller disk to a larger disk – but you should do that offline not live.  The examples here are all done using a live filesystem.  To do a non-live filesystem, boot into single user mode, mount the old drives as readonly with mount -o ro -a, then mount the new drive as normal and run the same commands to dump/restore.

Ok, so i'm going to assume that you want to dump the /usr filesystem into /mnt/usr (a filesystem mounted on a seperate disk)...

cd /mnt/usr
dump -L -0 -f- /usr | restore -r -f- Copy

This will dump ALL files in /usr into /mnt/usr.  Status updates are written to the screen every 5 minutes.

As this can be run on a live filesystem, you can run backups during normal operation (although the disk performance hit should be taken into account)

What is really useful is that you can pipe the restore command via ssh to restore to a remote server anywhere on the internet... an example would be...

dump -L -0 -f- /usr | ssh -2 -C -l remoteuser 10.0.0.1 restore -r -f- Copy

That would restore the files into the home directory of 'remoteuser' on the remote server.