Copy Time Machine Backups To Fireproof Safe Drive

Do you delete things from your Mac and think that they will remain on the backup?

Time Machine doesn't work like that. You should not depend on that. While they will remain for a while, that is what is removed when it deletes old backups.

Do you change versions of files over long periods of time?

This really is the only reason to maintain any sort of long-term historical backups.

Time Machine isn't an archival backup. It is designed primarily to restore you current state. It does maintain some historical data for a while, but you can never trust that it will.

If you need an archival backup that maintains every single thing ever on your Mac, Time Machine isn't the solution. I don't know what is, though.

Copying a backup makes no sense. If the backup is bad, then you just have two copies of bad data.

Instead of copying the backup, use that drive as a secondary backup drive in Time Machine.

Time Machine will alternate between backup drives if they are available. When you take out the drive and connect it to the Mac, it should run a backup on that drive. You can then disconnect the drive and put it in the safe.

Use Time Machine System Preferences to add that drive to your backup set.

Probably a better option is to use a different backup scheme like Carbon Copy Cloner or SuperDuper! to clone your drive to the fireproof drive. Not only will you have a different backup type, but you will be able to boot from it, if needed.

I don't know if what you wanted to do is possible. You could have to copy it to the new drive, then inherit the backups on that drive. I don't know if Time Machine would then consider it a different drive. It must, somehow, but I'm not sure how that would work. I've never seen the point in inheriting an old backup, as I explained, so I have never done so. It may work, but I just don't know.