I’ve been searching for the holy grail of Network Attached Storage systems and if what this wiki author claims to have done can be duplicated then I have just found it.

Basically, it will be a simple spare computer running FreeNAS with ZFS and Drobo-like sharing, swapping on different sized hard disks.

Want to use 4 different sized drives together as one large storage tank with 1 drive fault tolerance?Do you also want to be able to upgrade the smallest capacity drive with a larger one without having to move all of your data to a temporary location then re-build a new array and then copying all your data back?Ive figured out how you can do it.Ive figured out how to make FreeNAS using ZFS nightly build to use an array of 4 drives theoretically start with one drive but Im not going to talk about that here and allow the array to grow by replacing the smallest drive in the array with a larger one similarly to how a Drobo works. Im going to describe the process then Ill do a real world example.

via Freenas/ZFS and FreeNAS expansion – Wiki.mattrude.com.