well with a RAID 0 configuration, your data will automatically be backed up on the second hard drive. As you write to one hard drive, the same data will be written to the second. If the primary one falls, the other one will be there to back it up. Although with a RAID configuration you need to have a RAID controller, and the same hard drive as you currently have (i think).
And a backup disk should be placed physically away from the main hard disk depending on why you are backing data up. Generally this is the safest option becuase if something physically happens to your system, ie, short circuit (rendering everything in your system dead), power supply blowing up, or probably many other things that i wont list here, your back up is safely somewhere else.
It really depends on HOW much data you are backing up as to whether to go all out with another hard drive or just burn to CD or DVD.