Disk Utility and Raid

I have been trying to set up a very basic Raid mirroring system. Over the past 6 months I slowly ripped all my DVD's in order to play on an AppleTV. I have about 2.5 TB of data, all stored on one HD. I just purchased a second 3 TB HD in order to back up my first external. When attempting to set up my Raid disk utility states that all data on both drives will be deleted. This will not work. Is there a way around this? Any suggestions? I don't want to have to purchase another 3 tb drive in order to put my data on while I set up my Raid. There must be some alternative. This is my first time setting up a raid system and quite frankly until recently when I lost a bunch of important data I had not even backed my files up at all.