Being a “belt and suspenders” kind of guy when it comes to my data, I decided to start having Time Machine backup to two drives. But I wanted BOTH drives (TM1 and TM2) to contain the complete Time Machine history back to Day One.

If you simply connect a new drive (TM2) and add it to Time Machine’s list, the new drive’s backups will NOT contain all the history on the first drive. To get the history moved to the new drive, you need to copy the Backups.backupdb bundle from the first drive to the second drive before you enable the drive in Time Machine.

Finder does not seem to work for copying the Backups.backupdb to the new drive … in my case it took hours to “prepare to copy” only to subsequently refuse to copy them saying some items were “in use”.

SuperDuper, however worked fine to copy everything from TM1 to TM2.

The problem is, when I then added TM2 to Time Machine’s list, Time Machine was confusing the two drives. (TM1 would show up as both drives’ names, instead of TM1 and TM2.

The solution seems to be to do force one backup separately to each drive as the ONLY backup drive to “sync” the drive name and dataset, before enabling both drives.

Step by step (TM1 is the current Time Machine drive, TM2 is the new one):

  1. Turn OFF Time Machine
  2. Remove TM1 from Time Machine’s list (so Time Machine has no backup drives specified)
  3. Format the new drive, name it TM2
  4. Use SuperDuper to copy everything from TM1 to TM2
  5. Eject both TM1 and TM2,
  6. Physically disconnect TM2 so it will not mount automatically when you reboot
  7. (Reboot Mac just to ensure it’s in a fresh state)
  8. Mount TM1
  9. Leave Time Machine OFF, and specify TM1 as the backup drive
  10. In the Time Machine menu, do a manual backup “Backup Now”
  11. When the backup to TM1 completes, UNmount it.
  12. (IMPORTANT) Remove TM1 from Time Machine’s disk list
  13. Mount TM2
  14. In Time Machine specify TM2 as the backup drive (Time Machine is still OFF)
  15. Do a manual backup again, this time it will go to TM2.
  16. Mount TM1
  17. Add TM1 to Time Machine’s backup list.
  18. Enable Time Machine
  19. (to test, I then did two consecutive manual backups, ensuring that each time it picked a different drive)