x86/Sparc: ReadyNAS does not support JBOD, however, you can create a RAID 0 striped volume across all four disks or create a separate RAID 0 volume on each disk.
ARM: The Duo and NV+ v2 support JBOD. With the Flex-RAID JBOD option a separate RAID 0 volume is created on each disk. If you wish to remove a working disk from your system be sure to make sure your backup of the data on that disk is up to date first.