Windows Home Server. This is the exact set of problems that WHS was designed to address.

WHS:

- Provides data redundancy by duplicating specific shares that you set to be duplicated. It's a simple checkbox: If you configure a share to be duplicated, then WHS duplicates the data on the share to another physical disk
- Scales up as you add drives. Run out of space? Simply go buy another disk (internal or external), install it, and reboot. WHS will see that you've added another disk and it will ask you if you want WHS to use the disk for data duplication. Say yes and you're done.
- Installs a client on all of your PC's that will automatically back up your PC's to your WHS
- Allows you to remotely access your WHS administration screen via the Internet for troubleshooting / configuration
- Has a fair amount of 3rd party partners working on add-ons. For example, on mine I have a router control add-on that shows me a graph of my historical data usage of my Linksys router. I also have a bit torrent add-on that will automatically start downloading as soon as I drop a torrent onto a specific share
- Allows you to publish and share photos, videos, etc to the Internet

In my opinion, WHS is an awesome device. As I mentioned earlier, your problems were it's exact design goals. It was designed to address families with multiple PC's that need to be backed up, easy to add storage, and easy enough for an average user to administer.