When choosing a backup method, select the Sync option, here’s why. We want two identical drives to be the same bit-for-bit. If the main drive fails, we simply swap and use the synced drive until we rebuild a new one. No downtime. To achieve this, choose the Sync option. Syncing creates two identical copies that are updated when either file changes. If a file or change is detected on drive A, drive B is updated. If a file or change is detected on drive B, drive A is updated. If a file is deleted on one drive, both are deleted immediately, so be careful when deleting.
Learn more about syncing here: “3-2-1 Backup for Photographers“