you need 2 hdd's for RAID 0
So for example you have 2x 1TB HDD, turning on RAID0 will give you 2TB HDD with 2x the write + read speed but if either drive dies so does ALL the files on them.
Running the same drives in Raid1 will give you 1TB total capacity with the ability to survive if 1 drive dies without losing any files.
edit <fixed by Moderator>