Here's a spiffy way to do backups.
Create five-way ZFS mirror. Remove the front two drives of the mirror. When backup time comes around:
* zpool scrub
* perform system snapshot
* insert pair of drives from bank deposit box into disk array
* zpool replace the two disks, wait for resilver to complete - you now have a five-disk mirror in your machine
now, you can offline / store your data elsewhere.
* remove the front two disks. system operates in "degraded" state with three-way mirror.
When backup time occurs again
* zpool replace the offline volumes
* remove the next two disks
This allows you to rotate out disks, ensuring that they are spun up from time to time. zfs checks for checksum errors, helping you identify a bad drive before anything serious happens (also check SMART data).
I think this is best done with drives of the same size from different manufacturers to help reduce the chances of multiple drive failure.
I really wouldn't trust any other filesystem in a RAID. There are too many really good reasons to go with ZFS, and I've had some awful crashes with Linux software RAID.