Hello,
Since you are using a RAID 1 you can just delete the RAID 1 array and the information on the RAID 1 volume will be on both the hard drives that were part of it. However in this case since you accidentally selected the 500 GB hard drive, the information that was on it has been now at this point overwritten with the information on the RAID 1 volume.
The old information on the 500 GB could be restored with a third party software to recover files, check with a software developer that provides this type of applications to see if this would be possible.
Concerning the RAID 1, the information on the "non replaced" 80 GB hard drive will be still intact and you can just perform the rebuild again to the new hard drive that you bought, just make sure that you select it appropriately when doing the rebuild.
