Backing up the backup drive is one approach. Another approach is rotating multiple backup drives, which is what I do with Time Machine.
Time Machine makes it easy to keep rotated drives. If you add more volumes to Time Machine, it automatically alternates backups among them. For example, if you set up a second hard drive for Time Machine, it will begin alternating backups between drives 1 and 2. If you add a third hard drive, it will alternate between drives 1, 2, and 3. If any of them are not mounted, it will alternate among whichever volumes are mounted. This lets you plug in any number of Time Machine volumes at any time, and it’s not a problem (except for less redundancy) if only some of them are mounted.
There are lots of ways to do this. Over the years, as I’ve upgraded hard drives, I’ve ended up with a collection of older no longer used hard drives. I have reused some as additional Time Machine drives. Empty bay in a 4-bay drive enclosure or NAS? Stick an old hard drive in there and add it to Time Machine. Old Apple Time Capsule network backup device in the closet? Stick a bigger old drive in there, put it back on the network, and add it to Time Machine. Another old Mac still on the network?
Set it up as a Time Machine backup target.
If something terrible happens to any one of those Time Machine backups, there is at least one other volume with a history of recent backups on it, because of the automatic backup rotate feature of Time Machine.
I don't clone Time Machine backups because of a potential issue with the way Time Machine organizes backups. To keep a normal clone backup up to date in as little time as possible, cloning software such as SuperDuper! (or Carbon Copy Cloner, Chronosync, etc.) can scan the volumes, figure out which files changed, and change only those files on the backup. That way, a daily backup can take just 5 minutes instead of a couple of hours to copy the whole thing. If you use SuperDuper! to create and maintain a clone backup, then it would be easy to use SuperDuper! to backup the backup drive.
But that can potentially get weird when Time Machine is involved. Time Machine backups are stored in an unusual and complex way (e.g. lots of
hard links or snapshots to save space for multi-state backups). Also, they have special permissions to prevent tampering and corruption. For those reasons, I am not sure if cloning software can efficiently and safely update clones of Time Machine backups. It might decide to re-copy the entire volume each time.
