3 copies of the photo are made, then stored into drives D1, B4, D7. Any 2 drive of D1, B4, D7 can fail without data loss.
As soon as any drive fails, the SDS system will detect it, and immediately commence data recovery. A replica of the data in that failed drive is made from other available drives, and stored in the unused space of other working drives. Over time, the SDS system will settle into a stable state, with full Replica 3 restored.
Failed drives can be replaced any time. Once a failed drive is replaced (or a new drive added), the SDS system will commence to balance all drives, by moving the Replicas around, so that all active drives will have the same average utilisation.
The total useable storage space for Replica 3 is N/3 x C, where N is the number of drives, and C is the capacity of each drive.