The current set-up in my Mac Pro is a 250gb boot drive, with 2 500 gb drives as a RAID 0 for my FCP source files and things of that nature. I was thinking of adding another 500 gb or maybe one of the 1tb drives to the RAID, do I need to back up all the information currently stored on the RAID before adding the drive, or can I simply put in the new drive and add it to the current RAID array?
you can, however, stripe the drives in concatenated mode (JBOD), without the drives being the same size. you don't get any benefits of RAID1 or higher as far as speed and security but you don't have to worry about each drive only being the size of the smallest drive in the RAID. but even with that mode, you can't just add a drive to the raid, you have to unstripe the drives and once you do that the data is gone.