ZFS (or btrfs) will do that for you, no hardware RAID required. Most pro DSLRs have two SD card slots and will also do it (at the firmware level) if you toggle

ZFS (or btrfs) will do that for you, no hardware RAID required. Most pro DSLRs have two SD card slots and will also do it (at the firmware level) if you toggle the appropriate option (usually the options are RAID 1 or a unionfs-like "use both SDs" option).