You should use LVM and a cloning cron script.

I am going to assume this setup for you:

• 4 drives for LVM and file storage.
• External USB drive for backups. (at /dev/sdf1)
• No duplication -- use dd to completely clone as necessary.

LVM

You need to first set up LVM. This diagram is a good example:

Use a hard drive, and assign each to a LVM folder such as /lvm or whatever. This article is very good for describing LVM. I recommend reading it for a better and more detailed LVM introduction.

First off, partition all storage drives, but do not add a partition scheme! Use unallocated as the partition type. I will assume that your storage partitions are /dev/sdb1, /dev/sdc1, /dev/sdd1, and /dev/sde1. Your installation will be at /dev/sda1.

Next, you need to make the drivers LVM-ready:

pvcreate /dev/sdb1 /dev/sdc1 /dev/sdd1 /dev/sde1

Once that is done, you want to group those drives:

vgcreate homeNAS /dev/sdb1 /dev/sdc1 /dev/sdd1 /dev/sde1

This creates a group called "homeNAS" that conains all of the above drives.

Next, create the Logical Volume:

lvcreate --name share homeNAS

Create the filesystem:

mkfs.ext3 /dev/homeNAS/share

Once this is done, run the following to create the lvm folder:

mkdir /lvm/

Mount it using:

mount /dev/homeNAS/share /lvm

To mount it on every boot, add the following to the end of your /etc/fstab. Make a backup!!

/dev/homeNAS   /lvm     ext3       rw,noatime    0 0

Note that the above guide for LVM is very basic, and is not meant for fancy things. Read the linked article for a much more detailed explanation.