Hi, I have a suggestion or 2 for a fix or a work around. You might try formatting the new drive as a standard one first and then check to see if you can get an option to reformat it to Raid 1, but don't do this without backing up your data. If this won't work, you can just leave the setup as 2 seperate volumes and schedule multiple local backups to the new drive.
If neither of these is workable or acceptable and you have a seperate backup, delete all volumes and partitions from both drives and start from a clean, new Raid 1 setup. Please remember Raid 1 is absolutely not a backup and it protects you from exactly one "clean" hard drive failure. This assumes it works the way it is supposed to and allows you to rebuild the lost drive automatically when a new drive is installed.