Mirroring and system imaging are not the same thing.
Mirroring uses a RAID configuration like RAID1 or Microsoft's softraid function via the disk manager. In both cases two hard drives of the same type and size are needed and changes to the system will be written to all drives in the RAID at the same time.
System Imaging or Cloning will copy the contents of one drive to another. This is mostly used when changing to a new and bigger or faster drive which will become the new boot drive.