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I have some sensitive work data on my mac that i keep in an encrypted backup. It's a big file of about 19gb.

I can open and access it fine using the password i set, however, i have noticed that every time i do this and then open some of the files within and then finish and dismount the volume, time machine will begin backing it up.

However, it doesn't simply back up just the changes i made, it backs up the entire encrypted image again and so makes a duplicate of it, this is rapidly using up space on my time machine drive, is this supposed to happen and how do i stop it?

NB: It also does this even if i don't make any changes to the encrypted image and simply view some of the files within the volume and then close it.

Many thanks for any help you can give.

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Time Machine backups files as seen in Finder, it can't look inside your encrypted image. So every time you modify the image (even just the access date of some of the files in it) TM will register the image as a changed file and backup the whole image again.

You can exclude folders in TM preferences, so you could move your image into a dedicated folder and exclude it. And then use other means, e.g. a USB drive, to backup the image manually.

You could also use DiskUtil to create a new encrypted image as a "sparse bundle" and move the confidential content into this newly created image. Using the "sparse bundle" format should, in theory, allow for a more granular update record so TM doesn't need to backup the whole image each time. I had some mixed experience with this in the past though.

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