No mirror also called raid 1 need both drive of the same size. Also some users don't understand raid 1 really isn't for data backup. It's for hardware failures, so that if 1 drive fails then you can continue to use the 2nd one until the failed drive is replaced and the raid is rebuild. If there is a os failure, virus infection, or accidental deletion of file, then what happens to the first will also automatically affect the 2nd drive.
e.g.
If you delete a file then the file will automatically get deleted on the 2nd drive.
This is why raid 1 is called 'mirror'.
Just do a weekly backup or use a backup software to setup a schedule backup.