two pools, internalPool, externalPool
use ZFS send and receive to migrate your data from internal to external, you and do whole fs or incremental if you keep a couple of snaps local on your internal disk, this can get excessive if you have a lot of delta or you want a long time.
http://docs.oracle.com/cd/E18752_01/html/819-5461/gbchx.html
of course you will need a system that can use ZFS, there are more options for that than time machine, its block level and its fast, and it doesn't depend on just one device, you can have multiple devices (I like to keep some of my data at work, why? my backup solution is in the same house that would burn, if it burned...)