I am using Time Machine to backup an external HDD (A) (2 TB) linked to my computer to an external 6 TB HDD drive.

I now had to shift the data from HDD (A) to a new 3 TB one (B). Is there a way to continue the backup of the data so that Time Machine doesn't think it has to start the backup of (B) from scratch? Does TM automatically detect that the data on the new hard drive (B) are the same as the ones previously backed up on the old hard drive (A)?

Given how flimsy HDD are, I thought this would be a common problem. But I didn't find anything in the questions.


When you plug in the new HDD, do you get a message saying

Would you like this computer to inherit the backup history from xxx on the "xxx" backup disk?"

then an option to choose

Don't Back Up Now, Create New Backup or Inherit Backup History?

If so, click Inherit Backup History. If not, follow the steps below: Open Finder and navigate to your backup location

In terminal, type (but do not press return yet

sudo tmutil inheritbackup

Drag the backup location to the end of the terminal command, and press return for Terminal to navigate to the correct path.

If it was successful, it will ask you for your password. (It won't be shown in the Terminal window as you type.)

An alternate method would be to "Associate the OS X volume, which is:

sudo tmutil associatedisk -a /

You will drag (from the Backups.backupdb folder) the folder containing the backups from the old drive into the Terminal window for the path. Again, if prompted, enter the Admin password.

  • Will this work if the Mac has since been updated to a new OS? In my case I was using 10.6.8, got a new machine with 10.9.5 and want to continue backups on the same drive since it's barely 1/3 full. – iwantmyphd May 21 '17 at 21:18
  • 1
    This would be better if you showed the final versions of the commands rather than just pre-drag-and-drop – Andrew Wolfe May 11 '18 at 17:16
  • I'd just like to add that after performing sudo tmutil inheritbackup ... you will be told if the operation succeeded and returned to the command prompt. You will then still need to set up your Time Machine backup as usual in System Preferences but you will be asked if you would like to continue using the backup (or overwrite). – Ryan H. Jun 18 '18 at 21:52

To continue using the already backed-up data after moving the source data to a new drive you have to associate the new 3 TB disk with the old one:

sudo tmutil associatedisk -a "/Volumes/NameDiskB" "/Volumes/NameBackupDrive/Backups.backupdb/ComputerName/Latest/NameDiskA"

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