It's windows only, but storage spaces is a pretty good match for what you describe. There are still some rough edges, but it's overall a pretty nice setup for

It's windows only, but storage spaces is a pretty good match for what you describe. There are still some rough edges, but it's overall a pretty nice setup for me. I setup the virtual drive to require at least 2 copies of the data on the underlying disks (but there are options for 3 copies and more I believe), and then you can add and remove disks to the array kind of whenever you want. They can be different speeds, sizes, whatever.