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Here is the deal: I had an external 4TB hard drive with around 3.8TB of data which I included in my Time Machine backup. Since it was nearly full I decided to upgrade to a new 8TB drive and transferred all the data to that drive.

Of course I want to include that new drive also in my backup but I don't want to have the data twice on my backup drive (backup of old 4TB drive + backup of new 8TB drive). So how can I tell Time Machine that this new drive is basically the same as the old one only with a greater capacity?

Would it work to manually create the hard links to the files or would that just confuse Time Machine totally and destroy my backup? I tried naming the drives the same but when I start the backup it will just begin to transfer the 3.8TB, so the drives are somehow referred to by a special ID.

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1) Drives in Mac OS X are tracked and referred to in the operating system by UUID (you can find that information in Disk Utility by selecting the drive on the lefthand side, and pressing Cmd+I to Get Info)

2) messing around with hard links is a bad idea - Time Machine is already sort of a house of cards.

3) what I would recommend would be the following, which will take some time but preserve your sanity and the sanctity of the files in question:

  • allow Time Machine to complete a backup of the new data, even though it will duplicate data
  • add your original 4TB external to the exclusion list in System Preferences -> Time Machine -> Options button
  • with the original drive connected, "Enter Time Machine" from the Time Machine menu bar icon, and select the drive in question (Make sure you select the top level drive in the "Computer" section of Finder [Cmd+Shift+C with Finder active])
  • using the action menu (the gear icon in the Finder toolbar), select "Delete All Backups of..." to remove all backups of the 4TB drive and free up space
  • more info here: https://support.apple.com/kb/PH18855
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  • Yeah, that would be a solution, but that of course removes all previous versions of the files in my backup (which is actually not really a problem for that particular data). It feels like there should be a way to somehow "relink" the files... However thanks for your tip, I will mark this as the accepted answer for now. – YourMJK Aug 8 '16 at 3:13

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