You are incorrect. RAID 1 is a mirror setup. There are two drives with exactly the same information. One of the two drives is redundant. RAID 1 does not include striping and only requires 2 drives for redundancy.
I think what DDub is getting at is that there is no redundancy for the data received while the disk is mirrored to the new twin. For that, you'd need a mirrored pair plus a drive to yank out as the backup.