When asked if you want to replace the current backup disk or use both, select the Use Both option. This will enable backups to both drives concurrently.

To add three or more backup destinations, select the Add or Remove Backup Disk option. You may need to scroll through the list of backup drives to find it.

With multiple drives, Time Machine will make hourly backups as normal, backing up to the drive with the oldest backup first, so all drives remain as up-to-date as possible.

If a second backup drive isn't connected, Time Machine will continue backing up to connected drives, and will automatically update the backup when the drive is reconnected, perfect for off-site or long-term backups!