Question: How do I back up Time Machine backups?
Why: Time Machine backups was on an external drive that failed.
What I did: I replaced the drive and restarted Time Machine on it. Wanting to not loose months of backups again, I tried copying Time Machine files to another new drive, but got an icon telling me “it ain’t gonna happen.”
The partition on the new target drive is formatted APFS Case-sensitive, which is the same as the current Time Machine drive. I tried Disk Utility > Restore with Time Machine drive as the source. Failed.
Time Machine backups are on an external drive, which will fail, and I can’t find a way to back them up.
Don't try to back up a back up. It won't work. Just create two separate backups.
It is just a matter of disconnecting one and connecting the other periodically. A lot of people do it.
Some people like to swap out the backup drives daily, weekly or monthly, perhaps also storing the one not being used offsite for extra safety. Other people will have one at work and one at home, or some similar situation.
Thanks for looking into this, and your response, Gary. I'm disappointed that Apple hasn't provided elegant redundancy of Time Machine backups when the external disk dies.