Can you Raid a drive that already has data on it. I want to mirror the drive. Also this is my data drive that doesn't have the OS on it, can i run the OS on a single drive and raid the others?
Yes, if you have three drives you can run the OS on one and RAID the others.
If you want to mirror the drive, you'll have to format it if I remember correctly.
Remember that RAID1 is not backup...if you want to backup your data just copy it over to the second drive manually and leave RAID alone. Or use a utility like SyncToy to synchronize the two drives.