I have a lot of data (about 7 TB), stored across multiple hard-drives with varying sizes. I would like to have a backup of that data to be safe against drive failure. A RAID is not a good option for me, as I want to keep my cost low and be able to easily extend the storage capacity of my setup by buying an additional HD.

I remember seeing a piece of software that generates parity data over all drives and stores that on an extra drive. That solution protects the setup from hard drive failure and works with varying drive sizes (as long as the parity drive is the biggest one).

But I can’t seem to find that software again. Does anybody now what I’m talking about or have any other solution for my situation?