I am no RAID god, however, I am almost positive you can not make a RAID 1 array and have the data simply duplicated immediately. A RAID stripes the data across both drives, and once you set up the array, it sees it as 1 drive, so any information you put on it is split (or copied on a mirror only array), you would have to format the array before it was recognized.
However, I think you have an easier solution......if all you want to do is backup current held data...........just add another 500GB drive to your computer as a stand alone, and then copy all the data from the old to the new.....then pull the new one out and store it properly.
It doesn't sound like you need a RAID 1 array, since you already have the data you want to save. A RAID 1 is best served to a fresh system where data WILL be saved along the way.
Now, if you will be actually using this drive as you go forward as well and you want to have the backup for all your newly acquired data as well, as far as I know, you will need 2 fresh drives to create the RAID 1 array and then copy all the data off your single drive into the RAID, leaving you with 3 total drives. Or use a backup service to archive the data on your old drive, then create the array and format it, then pull down the archived old stuff......