First of all I would recommend that you use Raid-1 as your fault-tolerant hardware. RAID-1 is most often deployed with two disks. The disks are mirrored – providing fault tolerance. Read performance is increased while write performance will be similar to a single disk – if not less. A single disk failure can be sustained without data loss. RAID-1 is often used when fault tolerance is key and there isn’t an exceptional space or performance requirement. For the backup of domain controllers, you should use a dedicated local disk – schedule backup – and the copy the disk content to tape/secondary storage. You should install/promote another domain controller in the same domain for redundancy.
Your recovery procedure in case of a single DC failure in a domain with multiple domain controllers should involve cleaning the metadata, installing a replacement server, and promoting it to a domain controller (and seizing FSMO roles if required). As long as there is a working domain controller in the infrastructure, you should recover from an Active Directory domain controller failure by building a new domain controller, joining it to the existing domain, and allowing Active Directory replication to update it to the current state. The only time you should use domain controller backup images is when the failure has resulted in loss of all the domain controllers in the infrastructure or if one or more objects have been deleted from Active Directory by accident and need to be authoritatively restored.