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Starting from one TimeMachine 2TB external USB HDD filled to 1.2TB, I used the Apple-recommended "Drag Backups.backupD folder" to a second 6TB external HDD. Counting files (5M) took 2 days (!) then the copy (in progress) says it will take a day.

My objectives were:

1) Duplicate existing 2 year TimeMachine with a local copy in case of crash;

2) After copying complete, add second HDD to TimeMachine to run in 2-drive TM mode;

3) When 2TB original disk nears full, copy to another new 6TB drive and take the 2TB offline and use the two 6TB drives in the TM 2-drive mode.

My concern is: when the current copy to the 6TB HDD is complete, will MacOS be OK with adding the 6TB drive with the copied data already on it?

(BTW, I also have ARQ backing up remotely to Amazon AWS for off-site backup.)

Thanks,

Rick

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Because I think this might be a common question, I decided to test this while my terabyte drives were copying was: 1. I went to my backup iMac and formatted 2 small USB memory sticks - named TM1 and TM2.
2. Then I went to TimeMAchine Preference and under “Option” I setup TimeMachine to use TM1 and excluded the entire system drive and included a 3rd USB stick on which I had put a few files, about 375MB.
3. Then I allowed TimeMachine to back this USB test drive to TM1.
4. Then I disabled auto backups from TimeMachine and did the Apple-recommended “Drag Backups.backup folder” from TM1 to TM2 USB stick to copy the TimeMachine backup.
5. Then I went to “Select Disk Drive” in TimeMachine and added TM2. 6. Then I clicked “Backup now” and my test USB drive was backed up to TM2. I restored one of the files and then initiated another “Backup now.” This time the backup went to TM1 per Apple docs that the two drives will alternate backups.

So with these test USB memory drives, I think I have answered my question above. I have a duplicate of the historical backups before adding TM2 to TimeMachine and now successive backups will alternate between the drives. So basically an ongoing backup of my TimeMachine drives.

Now to wait for the “big boys” TB drives to finish copying to see if they will work the same way.

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  • Oh no! It didn’t work on the TB drives! It got stuck at the very end of the copy with 0KB to copy and 5 seconds remaining. I tried a computer restart, then restarting finder. The Backups.backup folder on the destination drive was grayed out and inaccessible, i.e., clicking on it did not reveal the underlying files. Did a Disk Utility “First Aid” check on it which came out ok. But the folder remained grayed out. Guess I’ll try the Disk Utility restore technique next.
    – rick
    Commented Jul 8, 2019 at 13:39
  • I finally used Disk Utility Restore option. It finished in about 18 hours - much faster than the Finder copy attempt above. Don't forget to right-click on the Restore destination drive when the Restore is complete and Rename it so the source and destination drives have different names. Then go to TimeMachine Preferences and Add the new Destination drive if you want to have TimeMachine alternate backups between the two drives.
    – rick
    Commented Aug 13, 2019 at 20:35

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