If you set RAID1 rebuilt to auto, whenever you replace a hard drive let say bay2, it automatic format the replaced hard drive in bay2 and then duplicate the data from the hard drive in bay1 to bay2. While RAID is in rebuild, it is very slow to access your data.
Let say you have a backup hard drive in the safe. Now both of the hard drives in your device fail. You can remove the two bad hard drives. Put the backup hard drive in bay1 and power on the device. The device will notify you your RAID is degraded. Now you can power off the device and add a new good hard drive to bay2. When power your device, it will automatic rebuild the data from bay1 to your new hard drive in bay2. If it doesn’t automatic rebuild the RAID then you can use the WD Utilities software to manually rebuild the RAID. As you know rebuilding RAID is very time consuming as long as there is a lot of data on the drive.