[Dean]

The best reasonable option for private parties is external storage that matches the capacity of their total internal storage of all their machines plus any archival storage such as CD/DVDs smaller external drives, etc. they use.

This gives them 2 copies of everything, one of which they can grab and run with in case of a fire, etc.

If you want to do even better, you can 2X or 3X the external storage and always have one off-site.

There are plenty of good manual and automated products/processes to keep everything in sync.

I have 2 mirrored 2.2 terabyte servers with internal RAID 5 disks. I keep threatening to move one to my Dad's in Gardnerville and setup the sync accross the Internet, but haven't done it.