Inherit Time Machine backup after changing source hard drives 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.