What will happen if I replaced one of the two 1TB HDDs with 1 5TB HDD, mirrored all the data of the other 1TB HDD, then replaced the other 1TB HDD with the other 5TB HDD?
I'm not even sure how mirroring works, is it like copying data?
You would still have a 1tb array and have 4tb unused space on both drives. Depending on the raid controller it may let you expand the container or it may not.